I'm not an expert on this, but I know that if you're concerned most (or maybe all?) school districts will evaluate young children for speech to see how they're doing and if they need services, and will even give those services for free (well, as part of your taxes so not free but you don't have to pay a fee.) If you call your school district, you might get someone to talk to him and see how he's doing.
I will also bump this up so hopefully you get more responses. Good luck!
I wouldn't be too concerned since he is talking. If you ask for something that has a set answer, just repeat the appropriate answer - would you like to go down? he replies up. Do you mean down? and show him what down is - or what I do with my 15mo DD is if she says up when we're already holding her, we lift her higher, which in turn gets her to say 'no, down" and then down she goes! I've seen later talkers say nothing one day then wake u speaking full sentences the next, it's not unheard of.