J.,
I know people will mean well by telling you that every thing will be OK, and I do not mean to be any kind of alarmist, but what if everything is not fine, and you wait and see, and he could have been helped with early therapy? You have zero to loose by getting an evaluation from a speech therapist right now, and that is the professional you should be asking!
Developmental issues are moving targets, and the earlier you get therapy, the more effective it will be. Right now, he is learning not just language skills, but how to interact with people based on language skills, so one delay will effect anohter delay and so on. His growth right now is astonishing, and it is all interdependent on language.
Time is free, and it is on your side right now. Take advantage of it, and hope that the therapist scolds you for being a pushy Mom with a kid who has no problem whatsoever who will some day be in the gifted program at school, or a brain surgeon, or a...what ever! The better news is, he just might still be all those things even if something is wrong, but only if you get him the help he needs as early as you possibly can.
My point is, don't risk it. Get an evaluation instead of waiting to see if he turns out like your neighbors nephew's grand son who did the same thing and is fine now...it is worth your time to find out from an expert when ever you are talking about developmental issues. They are too important to wait and see.
M.