TBH, while your DS does seem to sound advanced compared to a lot of children (my DS started recognising 'O' and 'S' about a month ago at 21m), I don't think there really is a case of 'advanced' or 'behind' in children so young. It usually all evens out by the time they've been at school a couple of years. (Though in reading, your DS is on the early end of the spectrum.)
By all means be proud of your DS because it sounds like he'll be an early reader - something which can only help him at school and in life generally, but I wouldn't worry about finding particular activities at this stage. His attention span probably isn't all that great, so dedicated activities are likely to end in mishap.
Mind you, saying that, since my DS began recognising characters, I've started drawing them on his blackboard (I started him holding crayons and chalk ridiculously young, and he seemed to enhjoy it so we persisted), and he will say the ones he knows, though often as a 'bye "O", bye "ssss"' as he carefully wipes them off! Still, goes to show how contrary they can be.
Also, don't do what I did and instantly break out the flashcards (which my mum bought be as she only sees us every six months to a year, and used to be an LSA - I'm not a pushy parent, honest!), because he will probably just happily sit and fold them in hlaf or otherwise crumple them. At least she only got him some cheap ones!
And be prepared for him to possible stay at recognising the odd letter and not make even progress - my son hasn't really learned more than three letters, and has stayed that way for the last month. Boys do apparently do things in fits and starts, another reason to simply praise him when he does it, but otherwise not read too deeply into it.