Hi,
First of all, you need to know that it has nothing to do with your parenting or failing as a parent. My sister-in-law is a moron (sorry, but she is), she has no parenting skills what-so-ever, let's her kids do whatever they want and they are both smart as a whip!
I have a 4 year old daughter with PDD-NOS and my 2 year old daughter has pretty severe autism. From what I read in your post, it sounds like your son does have some words and you also stated that he "mimics" other children. That is a WONDERFUL sign in itself. My 2.2 year old doesn't have any words at all and will not even go near another child or even her own sister, we can call her name 50 times and she won't look and she just gets fixated on having 2 objects in her hand at all times. We do have her in pre-school but it's only 4 days a week, 3 hours a day. We are going to see a DAN dr. next week and maybe start chelation but your son sounds like he's on the right track.
I would definitely keep up the speech therapy and the fact that he follows other children and copies what they do is an awesome thing.
Keep your head up and definitely keep a positive attitude, our children might not talk well, but they can sense when we are upset or worried. I think they're all smarter than we give them credit for
Good luck to you and your family,
Lisa