Sports. Find one for him. My son was similar to yours. Very bright. Shy. An easy target for bullies. Boys seem to compare themselves by sports ability. If he'll play T-ball, start there. Soccar is ok too.

Some kids need to be taught social skills. He may be very bright in some areas but not all. Lots of books out there- many are good- in our area the school calls it "skill streaming". Specific lessons on how to learn social behaviors- taught with role playing.

Find an interest of his. Expand on it- trips, events, etc. Find a classmate who likes the same thing. Invite that child over.

My son was very interested in Teddy Roosevelt. In third grade he found a boy who liked him too. We went to the rough riders event, Roosevelts home, etc. That one common interest was the foundation for a long friendship. The shared experiences increased their bond. One friend tends to lead to another. Eight years later, they may drift away for a while but when things are tough they get back together.