My memory of the time was coming out of the late hippie era with the iconic pure bell-bottom jeans which were usually tight above the knee, then belled (not flared) out to usually past the shoe tip. So tight above the knee, belled below. Naturally (like that or not) "tight" was tighter in women's bell-bottoms than men's.
Starting into my high school years bell-bottoms were done, but a more reasonable, and very comfortable "flare" style had evolved from them. The flaring seldom reached the toe of your shoe, usually just covered the laces.
But a few years later, even before I graduated, flared jeans were out. What replaced them were the straight-legged jeans referred to in other answers. By the time the 80's began, they had a corollary for people who just weren't going to wear those uncomfortable things called boot-cut which were also basically as described above in other answers. Straight-leg jeans were called "jeans" by most people after flared jeans went away, while boot-cut jeans were called just that.
Both styles had more room in them above the knees than bell-bottoms or flared jeans did. Levi less so than Wrangler, to the point I found Wranglers very uncomfortable as nothing was held in place. Unlike Blowfly's Rollo, I did not like a fresh pair of baggies.
At the end of the 70's designer jeans had not become much of a thing, at least not in Ohio. I believe that's because they had to evolve in response to the late 70's standard jeans having more room inside above the knees. A couple years of that and women (just doesn't seem like a male-driven demand, does it?) demanded more form-fitting jeans. For the obvious reasons. And designer jeans stepped in to handle that.
DJ's were much tighter above the knees and given what they were responding to (my take anyway), they chose to be tighter than the straight-leg jeans. Bear in mind straight-leg jeans are still "frumpy" in that they usually can get pushed up some, crumpled might be a better word, right around those delicate ankles. As hard to see a girl has a well-turned ankle wearing that kind of thing as it is to see she has a nicely shaped bottom when there's intentionally extra room there... (think: the difference between flat panel pants and pleated pants... mom and grandma wear the pleated pants). And given that they were female driven to be tight anywhere real form desired to be shown off, they just kept getting tighter. About 1989, I picked up a couple kids, cousins of my boss, to bring them to an outing and they somehow got on the subject of their mom's "fat jeans", jeans that weren't tight much, said they were OK when I asked if she'd been wearing them just then when I picked them up and that I thought they fit her very nicely, just not like her good ones that didn't make her look fat at all.
My bet is that Lisa wasn't the only woman calling some jeans fat jeans and others being much more her going out jeans. 2005, for sure? I wager "skinny jeans" as a name took about that 16 years plus another 2–3 before 1989, to develop out of "fat jeans" for non-tight ("non-taut" might be better), and be given an actual name that suited the concept. And "fat jeans" would have disappeared then.
Back to the 70's though, in Ohio. In my experience, anything store brand or close was just "jeans" and as best as I remember, they were fairly tight. Some had flares, but still were fairly tight above the knees, maybe to save cloth/cost. Until flares had died out and straight-leg jeans took over, whereupon straight-leg jeans got called "jeans", generically. I truly remember nothing in the 70's as tight as skinny jeans, and the only things as tight as designer jeans would be in the 80's were store brand jeans.
The upshot is, at least from the standpoint of what I saw in Ohio and on TV, there were NO skinny jeans in the 70's, neither by name (of course... 2005) nor by actual construction. There WERE tight ones, but just that, tight. Not meant to stick to the body. It wasn't until the mid-80's anyone even talked about "painted on jeans" (whereupon a couple of men's magazines literally did that, and then it became an occasional artist thing).
My take on what drove it was female desires. My wife is not alone in having answered my question about "Would you rather have comfort or look good?" with the latter. And then prove it. Women will also spend far more than the average man on a piece of clothing. Those two factors, along with a couple years that must have been horrifying to them in which the jeans that were tight above the knees disappeared and only frumpy ones were available to most or all of us, created the designer market, I believe. Until that existed, there had been tight, in the places of greatest interest and for men as well as women, jeans, but not tight everywhere. I have no memory whatever of anything really approaching "skinny jeans" in the 70's. Even the old hip-huggers didn't qualify: they were more about lowering the waistline a lot and having to be very tight to stay in place, though doing double duty as a great reason to have them, and being tight on the thighs too, but out of habit so-to-speak. Without them, the designer jeans market might not have taken tight to its "logical conclusion", but they were not in any way skinny jeans, especially since they fell into the bell-bottom era and had belled legs.
The closest I remember in the 70's would have been Capris, very, very tight all the way down, but almost never made in jeans since the point was the tightness and jean material of that day was just too solid to really be tight like that. Brushed denim (mid-70's, dead early in the 80's) had to be developed before Capris could be made of anything considered denim (brushed denim was all that denim-like, really). If I were to guess, as designer jeans makers tried to seem Euro even if they might not have been, and Capris were definitely fairly gone as a major thing in the US by the end of the 50's, though never gone altogether, not at all, that maybe Capris were still in in Europe, and gave the "designers" an inspiration that lead to the ludicrosity of skinny jeans.
Don't get me wrong. I love a woman's form being utterly on view. I love a woman saying "I only wear pants to keep from being arrested, so I wear something that makes me just as naked." But skinny jeans look ridiculous.
Just like head hair has a purpose beyond wacky styling (it helps give one's head a more pleasing shape, if one's head is... unfortunately shaped), pants have a similar helping hand to offer. Or both, of course, can just be worn without that concern.
And I just flat do not remember them in the 70's at all. So I think there isn't a great true name for you to use in the story. The best I can suggest is a tangential reference, something like "If he had the hair and 200 extra pounds, his jeans would let him look like Elvis. He's even got the sneered up lip, from regarding the rest of us. He doesn't seem to get that the odd man out isn't everyone he sees, it's the odd man, and man, when it comes to odd... !"
As you can tell, not an author. But I did live through the 70's and just don't remember any skinny jeans or anything like them.