I've been here since my daughter was 4 months old *blushes*
I know it is kind of silly, but at the check-up they said she was one of the most advanced 4 month olds they'd ever seen. My daughter was similar at 16 months with the letters, etc.(she's now only 21 months) Instead of speaking, though, she went nuts with sign language and learned all the signs on the 30 signing dvds we bought her. We didn't have any more signs after that. She's just now starting to try to use her words, instead of just signs.
Anyway, I get why you are thinking about this. I have learning activities for my lo, and she loves working on them. I look at the skills she's currently working on and then I provide specific activities that she can do - but that actually challenge her in that area. It's really rewarding when she asks for them over and over and I see her slowly master them.
You might look into Montessori-inspired activities if you want to do things at home.
Oh, and don't worry about labeling him yet. You know he's advanced... just meet him where he's at. Just give lots and lots of love and empathy. When he throws a puzzle piece at you because his fine motor skills aren't up to where his mind is, give him a hug and help him achieve his goal. You can understand and empathize with that frustration.
