Hi B.,
I'm no expert, and I had all girls--who talked up a storm very early. I guess there are a few observations I made while watching them learn to speak--because I was fascinated and am both an educator and counselor, I just really paid attention to how they developed this skill.
1) I always spoke to them in full, simple sentences--articulating the words clearly, but never using "baby talk". I am kind of assuming you are doing this, too, especially since you have concerns based on your daughter, but thought I would mention it. I noticed children all around me tended to early speech that very much immitated how they were spoken to--which is not surprising, given that's how we learn language.
2) The younger siblings were a bit slower at speaking themselves--not a lot, but it was clear they "used" their older siblings for communication help when possible.
3) Language capabilities seem to have a genetic predisposition. In my own case, my daughters were very articulate, very early--and I was, too, as I hear from my parents. Since autism can have a genetic predisposition, it might be wise to continue having your little boy checked out regularly, even though there is a good chance he is just fine.
4) Here is where I get a little "theoretical"...I noticed with my youngest daughter that she began by babbling in what sounded like complete sentences. She had intonations, and clear beginnings and ends, with questions sometimes. But there were no actual words that were recognizable. As she began to develop further, vowels worked their way into the proper places, but without correct consenants. The consenants developed last. By the time she was speaking English, we realized in that last coming together part, that she had been talking all along--just not clearly or understandably. So...does your son babble alot? Or is he mostly quiet? I think that would be more of a concern than not having specific words. That's just my theory, though, and not backed up by any science.
I do hope you find that he is just fine, and I think there is a very good chance of that. But because of your concerns, I would also listen to your intuitions and have him regularly checked by a speech expert. There are other things that can cause delayed speech besides autism, and most things, if they are caught early, can be remedied.
I send good wishes your way, and hope you had a happy Mother's Day!