Mk, you've gotten good advice on this thread. With a kid on the spectrum, you do him a huge disservice to assume he can't connect just because he is too smart for other kids. Kids on the spectrum at this age lack the social skills to play well or "click" with other kids. The example of wanting to measure first before building, while the other kids run off to build, shows a lack of flexibility, an adherence to his own way of doing things that prevents him from going along with the others. The complaint of being "bored" is also very typical of autistic kids-- "bored" often being a stand-in for "I don't know how to engage with the friends or activities that are on offer, I'd rather be off doing my choice of play by myself." I would take that as a cue that he needs to expand his interests, and be taught to get interested. Not a cue that it's all below him.
Young autistic kids often find things to be as important or more important than other people-- that is, the legos, or a particular arrangement of blocks, may seem more important than the friends. Preschool is a very, very good time to work on both flexibility and friendship skills-- other kids will make leaps in this area, and it will be hard for your DS to keep up, but a good foundation is essential.
Polarbear's suggestion about the allergies is great-- providing all food for him seems like the best way to keep him safe. I know another parent who went this route, and it was very successful.
DeeDee
