While I think 'they'll find their tribe eventually' is probably true in many cases, I'm not sure I'd rely on this when you have a potentially unhappy, isolated child. It's great if that child can find others who they connect with and share interests with, but often what they really need at school or in other situations is just some people to hang out with - play with at break and sit with at lunch. They don't need to be best buddies, just rubbing along will do.
That's why I do think it's important to teach children to rub along with others. Social skills are just that - skills. They can be improved with practice. The problem with having a potentially socially isolated child is that they may not get the same opportunities as other children - parties, playdates, playground and park trips - to work on and improve their social skills, and learn the give and take that comes with being in social situations.
I keep wondering maybe if he had better clothes or cooler toys or gave out sweets after school
I wouldn't do the sweets thing (he shouldn't have to 'buy' friends and that one becomes tricky and potentially exploitative in early secondary school, when they're out of your control), but essentially one of the things you want to encourage him to do is to be a game-creator rather than simply a follower or a joiner-in. So some really cool toys might help to change the dynamic - a remote-controlled car or plane, a giant bubble kit, a rocket-launcher. Then at least he'd have something to focus on at the playground or at parties if everyone else is ignoring him, rather than feeling like a spare part, and it might encourage others to join in with him, rather than him wanting to join in with them.
My DC can occasionally become overwhelmed in busy social situations like parties so we usually take a sticker or puzzle book. If DC doesn't want to join in the activities, they come and sit with me. That might be an idea for parties if he's on his own and feeling a bit down.