At 6 years of age, I think that you are expecting too much in his social skills. Our son was started to school when he was 6 in Nov., and he was emotionally immature, sweet, but shy. He ended up taking the 3rd grade over as he regressed to where he couldn't even add 8+1, at the black board.
With a new school, a new sibbling, that is a lot for a 6 year old to deal with. I would just be there for him, supportive, NOT baby him, but try to LISTEN to him, if he can share how he feels, although he probably doesn't even know how he feels. Love, support, not too much focus on professionals that will highlight his "problem", make him feel "different", jut trust your own instincts as a caring mom, but try to give him his own coping skills---maybe he is just shy, not all kids are social animals, you can't force people to be happy, or to be acceptable to others, but I think with his learning to cope and accept others, then they will be normally attracted to him, even though he may never have boo-coos of friends, he many not need them, just one or two good buddies.
Maybe you could have one or two little boys over after school one eve, leave them alone to work on their own social skills. Too much interference from mom may just heighten his own anxieties and he may grow to feel like he does have a real problem that makes him "different", and he will always have that stumbling block in his emotional make- up.
Just be there to listen, hug and love--some---and let the little boy grow up and develop his own set of coping skills.
P.