my boyfriend's manners

my boyfriend is a very polite guy and he's very sweet! but whenever he comes over and goes to leave he never says thank you to my parents for having him over, he's very nice to them and says bye but he doesn't say thank you. my mom knows that maybe he wasn't taught to say thank you even though it's the right thing to do, but I don't want my parents to say anything to him next time he doesn't say it. I also don't want to flat out tell him to say thank you, I tried to hint towards it by talking about an ex boyfriend of mine, whose parents never liked me, and I said I always said thank you to them whenever I left their house. please help!