If you were not comfortable with the evaluation the child study team did on your child when they prepared his IEP you might consider another assessment. For example, you could consult a developmental pediatrician. But the speech by itself, even with some motor problems, is not enough to suggest autism. A poor diet can cause all kids of problems, but we have no research suggesting that autism is one of them. My son turned 9 today and still has a very limited vocabulary.
Early language does seem to predict later outcome.
But children with delayed speech can still learn a great deal. If speech is coming slowly for your son you might consider giving him an alternative communication system. The PECS system developed by Andy Bondy and Lori Frost is one approach. There are also small handheld communication devices that can be helpful. Some of our older students use those small computers and you can tell how much they value them by the way they always keep them close by to use when they want to express themselves.