I agree with other PPs. I'd ask the school to evaluate your son as well as ask your pediatrician. If there is an issue to be addressed, he'll need an IEP.
Just so you know, there are a lot of reasons for this behavior other than autism. He could be more introverted, socially immature, anxiety, and often kids with speech delays have a difficult time developing social skills b/c they're not able to express themselves or pick up on what others are saying to them.
I'd continue with one on one play dates if you can. Social skills are just that--a skill. For some kids, they come naturally whereas others need practice even if they don't have LDs. I like this book b/c of the handy checklist in the back about how to have a successful playdate:
http://www.amazon.com/Its-Much-Work-Your-Friend/dp/0743254651/ref=sr_1_2?s=books&ie=UTF8&qid=1411077018&sr=1-2&keywords=rick+lavoie
It's also worth exploring why he doesn't like school. I'd consider talking with the school counselor. Sometimes they have social skills groups for kids who have difficulty adjusting. Good luck to you; hope you find some answers.