Has he shown a preference for any specific orchestral instruments? If he doesn't really want to take up another instrument, how about choral singing? If he has guitar lessons, does his teacher know of any guitar groups in the area that he might be able to join?
Its a bit stereotypical, but some instruments do seem to attract different character types - trumpeters tend to be show-off extroverts (who can count lots of bars of rest and then come in at the top of a very loud chord), oboists are a bit more introverted, violins (especially 1sts) and flutes will have quite a lot of exposed passages, cellists tend to be happy in a more supporting role (which is me - I love having mainly harmonies with occasional tunes, and the thought of a trumpet or piccolo solo would have me trembling in fear!)
Its also a question of what he feels comfortable holding - I play the cello, which works quite well with my long arms, but violins just feel like toys (although that's possibly because I've been playing a cello for >25 years). I can get a tune out of a flute, but nothing out of anything with a reed on it. Trombones are fun, brass with valves just gets me confused
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It might be worth talking to your county music service and seeing if they offer or are aware of any days where children can go and find out about different instruments and maybe give some a try. IIRC the orchestra doing Peter and the Wolf at the Barbican last Xmas had a couple of free sessions where you could do exactly that - so worth keeping your eyes open. If he likes the idea of brass then I understand the Salvation Army bands can be really welcoming and helpful.
I hope you can find something that inspires him, or a way to make his existing music more sociable. I wasn't the most extrovert child and music was a way for me to get out, join in a group activity and feel part of a team :)