This is Zhao Bing, Emperor Bing of Song, the 18th and last emperor of the Song Dynasty. Is this headgear a Tongtianguan? And whatever type of headgear it is, what more do we know about it? What does it symbolism and who was authorized to use it?
Update: I found these two great resources but still can't figure out exactly which one it is: A list of Guan, Jin, and other headwear, and A Social History of Medieval China, pg. 7.