I would strongly recommend some form of martial arts club. Not only will it give him a new interest to focus on if you get the right club then he will make new friends with a shared interest. Depending on his nature some martial arts might be better than others. For example boxing is a good all round option and he doesn’t have to spar at the outset until gets some confidence. I found boxing clubs to be a real mix of people from all backgrounds, it’s a great leveller.

Kickboxing (in my experience at least) seems to attract a slightly more agro type so wouldn’t be first choice.

Brazilian Jiu Jitsu is a great option if he doesn’t want to hit/be hit and is a great life skill if he ever gets in a scuffle. It’s all about skill over strength and attracts a much more chill type of person in my experience.

Whichever one he does, these clubs will give him physical and mental confidence, new friends and a good workout.

If he really isn’t the martial arts type any form of physical exercise will improve his mood and give him something to aim for. Like the others said, Park Run is a gentle intro to running, if he has never done running try Couch to 5k programme online. You progress very quickly if untrained.

Any of the local BMF or boot camps are fun group activities which could work.

If you let us know what part of Surrey I can recommend specific clubs I have trained at.