How about setting up a few playdates and see if he can make a friend? Has he mentioned any of the children? He may not find a friend on his intellectual level but he may find somebody he likes to run with, go to the playground, and do all the other things 4 year olds may like. You may want to go and volunteer/observe the classroom to see what's going on there.
I think PreK is NOT needed. If he is truly unhappy there then I don't see a reason to keep going. He is 4 after all and there will be lots of things he will have to deal with in K. Let him have another free year if you cannot make it work.
Last year when DS4 (3 at that time) didn't want to go to Montessori I let him stay home mostly because I saw the reason why he didn't want to go. I was ready to pull him out. It didn't happen at the end though. He kept switching between going and staying home and his older brother was at his class. Every time I was ready to pull him out (towards the end of oh so many months) he decided to go back.
