Could you maybe invite someone from school round for tea one day a week?
Or organise some dinner and a play at your house on the weekend for a few hours?
I would definitely get him involved more with groups, have friends over etc.
Hope it passes quickly for him. My ds said he had to sit on the bench the other day as no one would play with him. It's heartbreaking. He has a lovely group of friends and it just happened that two of them were off and the other two wanted to play alone.