Sounds fine he has great social skills and great understanding and comprhesion. Maybe he is just having some oral motor skill dificulty, take him to speech therepy for help with speech but it doesn't sound like autism to me :)
That should say my son was 36 months old when he started talking.
My 3rd Child didn't speak a word until 26months of age when he suddenly started talking in full sentences. He is now 6 years old about to finish up kindergarten and reads on an advanced 1st grade reading level. Some children just sit back and observe while they soak it all in. I would say encourage him to attempt speech (make him ask for "ore" and correct him "more" before you give it to him), try to introduce new words (repeat "cup" a few times before handing it to him, try and get him to say it but don't force him) and TRY not to worry to much.
Two of my grandchildren did not speak until they were three. There was nothing wrong with them, and when they did start speaking, they made up for lost time.
When my stepson started living with us at age 3 (he is 5 now) he didnt speak much either. In our situation, the reason why he wasnt where he was suppose to be was because his biological mother (she doesnt deserve to be called a mother) didnt spend enough time with him and he was always with some type of babysitter and of course babysitters make it easy for the child to get whatever they want without them even having to speak (i.e crying for a game/food)
However, after being with me 24/7 for two year he now knows how to write, read, speak spanish, count, etc.
Now you said hes in preschool...that could be a reason as to why he hasnt developed much. A lot of daycares dont give the children the individual attention that they need in order to learn. I mean its usually 10 kids to one adult.
I dont think your child is autistic..some kids just develop at a different pace. It takes a lot of work and repeating the same things everyday for a while in order for some kids to learn.