I would suggest keeping your family out of it, you don't want to create undue angst assuming this relationship lasts.
I would clearly say to him that it is embarrassing to be with him when he smells to bad, that everyone notices it, is isn't as subtle as he might want to believe. Point out that strangers notice it and actively avoid him/look at him with disgust (which I imagine is probably true).
Incidentally, how does this kid ever expect to be employed? I can't imagine anyone who reeked ever getting a job, who would want to be sat next to that?
You've tried being delicate, I suggest you give him a wake up call that he needs to grow up and deal with his smell.