I hope i helped a bit....
I know how badly they feel when their friends wont play with them,but i guess it is all part of the growing up and getting out into the world process....
Having some boys round from school sounds like a great idea!! Definitely something to pursue anyway....The different classes thing sounds good too....
My DD6 is currently really mad at her teacher,so we have problems here too....
Im so sorry your DS feels bad about it all,and hope it works out for the best in the end....Maybe if he is a bit clingy to this friend it might be the reason the friend is trying to distance himself?? I dont know,just chucking a couple of ideas around that you could talk to your DS about....
Good luck!!