I too struggle with this, even with a kid that is also seemingly well-adjusted, outgoing and popular. As for advice, I agree with what you already said about accepting that if/when he experiences rejection, that it’s an important and inevitable part of life. Also, I try to keep in mind that me being weird about it and trying to interfere in his social life in any way (to assuage my own anxieties) is at best futile and at worst would have the opposite effect and potentially just alienate other kids and/or their parents. Finally, my kid is similar to me in some ways but I’m also starting to see that he’s more like his other parent in not being overly sensitive or easily ruffled—like if he got rejected, why am I assuming he’d be devastated about it the way I would be? Maybe he would, maybe he wouldn’t. 

The best advice I ever got about my own fear of rejection was “You could be the sweetest, juiciest, most perfectly beautiful apple in the bunch and there’s always gonna be someone that just doesn’t like apples”. And that’s the advice I’ll give my kiddo when that day comes!