Calendargirl
Can he afford to host these gatherings? Is money a factor?
I thought that might be an issue, too. Some young people - who don't appear impoverished on the outside - are struggling. I wonder if the OP has thought of that - the 'potluck dinner' serving size indicates it might be?
If it were my son, I'd ask him outright and offer to help out financially with social events - but I know in advance that he wouldn't take offence at being asked. Some young men might not be happy with such questioning though.

