I would consider carefully what your son's speech pathologist is saying. She has training and experience and get paid for what they do and know. If you don't have to have him in preschool, I'd not put him in preschool. Learning to talk is very important. He will have difficulty with the other kids since he's both shy and speech delayed. You want him to have success with his first attempt at learning social skills. Otherwise, shyness is more apt to be a life long difficulty.
My grandson is still unable to speak clearly at age 7. He's been in special ed for the past two years and was in day care before that. He's definitely not shy but he does have difficulty getting along with children. It's my thought that learning to speak takes precedence over learning social skills. Once he's able to talk and relate to children with words he'll learn how to relate to other children. He may need help learning that too but you can help him with social skills at any age. Now is prime time for learning to talk.
If you're wanting him to learn some basic school stuff, such as colors, letter, numbers, shapes you can teach him those things at home.
My grandson went to a therapeutic preschool that is a part of the school system. He did well in that setting. The school system had him in Head Start before that and it was a disaster. He did not get along with teachers or children. He was so frustrated at not being able to get himself understood that reinforced his tendency towards hitting and kicking.
Check with Beaverton Public schools and see if they provide a special preschool (not Head Start) for children with learning difficulties. That might work for your son. But I suggest, since he's so shy, it might still be best to keep him at home, or if you need day care, in a private home with just a few children and a warm and supportive provider who will work with the therapist.
Putting what your son learns with the therapist into practice, daily, is also important. I know you can do that during non-school hours but I suggest that the consistency of home will help him more than preschool.