Hugs, Sunshine4. I know it's tough.
One thing that did help in our case was recognizing DS's perfectionism issues. He really didn't want to play with the other kids when he felt unsure of himself about something. For example, a lot of his classmates were playing basketball at recess when DS was younger. DS refused to play because he said he was "so bad", so we signed him up for basketball clinic to get him some confidence, and then he started playing sometimes.
I think DS being a loner on the playground was tougher on us in some respects than on DS. When he was in K/1st, I think sometimes he didn't mind spending time alone on the playground. He had issues with becoming over-stimulated in the classroom (he had a lot of trouble tuning out all the noise/action), and I think that sometimes, he used recess to recover from that.
Just some thoughts in case they might be helpful.
More hugs, Sunshine!
