K.,
I think it is so hard to tell if one child is "falling behind" in something. Kids are SO different. I have one nephew who is 2 1/2 and he can carry on a full conversation, and another nephew who is 2 wks older than the other and I can barely understand a few words he says. My sister has taken him twice to get him checked and they say he is fine (he just has problems forming the words in his mouth). Kids just learn things at different paces. Your son could just be focusing a lot on physical acheivements right now and will catch up on language when he is ready. Also, it could very well be his ears. My 11 mo old just had tubes put in her ears for constant ear infections over the last three months and she has been "talking" a lot more since! But it NEVER hurts to have him checked!!! It is free, and if there are problems it is best to catch them early!
Hope this helps!
S.