Madison is 21 months and 'dresses' herself every morning, but can't get tags in the right spot on her own. I just lay it out in the 'right' way, so that when she puts stuff on the tags are in the right place. She does shirts, pants, socks, shoes, and her coat, hat, and mittens but I have to help if the shirts have buttons or snaps in the back (as most things do in her size). She really doesn't allow any help so we just get up earlier so she has time to do things herself. I wouldn't have necessarily thought to do this on my own at this age, but her school is very pro-independence and they encouraged it (taught her how to do the coat flip) and sort of have a dress code, kids are only supposed to wear things they can do themselves-no onesies, supertight clothing, pants with snaps in the crotch, etc. It's really helped and now she insists on doing everything herself. What I find works is that when she says "mama do" or "mama help me" I'll come watch her and tell her what to do next and then she usually finishes it on her own. Oh, and she has a pretty big head too so I know what you mean about having trouble fitting shirts over it. Usually I have to help if there are no snaps or buttons, otherwise she gets frustrated and cries cause she thinks she's stuck in the shirt.
Maybe a way to help him with the tags, at least for shirts, is to teach him to put them on using the 'coat flip'. Seems like it would work the same.