He was himself an escaped slave, so at the very least we know he was in favor of abolition of slavery in that one case.
We actually only have two writings from St. Patrick himself. Most of our material about him comes from other sources. So limiting yourself to just those two writings, while indeed much more historically sound, is a rather drastic limitation.
However, since there are only two, and they are public domain, the analysis isn't tough to do.
This mentions slavery as it pertains to himself several times, but nothing useful about it in general.
A Letter to the Soldiers of Coroticus -
I think the following passage strongly implies that he considers making slaves of Christians and anyone who wasn't born into that condition to be immoral:
For they have been taken far away and abandoned in a land where sin
abounds, openly, wickedly, impudently; there freeborn men are sold,
Christians are reduced to slavery, and worst of all among the most
worthless and vilest apostates, the Picts
But again, these are the only two surviving examples of his writing. We know of more from his biographers, plus many speeches and sermons that did not get written down at the time. There is even some known early third-party biographical material that has been lost (eg: The Book of Ultán). This material is all inferior to the above two first-hand accounts of course, but in History, particularly Medieval history, you have to do the best you can with what little you have.