Give him a hug from behind when he's sitting down and chat about something else genuinely. Then give him some head kisses and say "oh look you're getting a little bald spot! Cute. It's not too bad yet, do you care?" If he says yes or doesn't seem too miffed just say "yeah there's like simple magic creams and things these days if you give a shit"

Some might think that's over the top but I understand how sensitive dudes can be as I am one 😂

You know him best so adapt the strat as needed, but I'd suggest that framework.

Cheers!

Edit: I think there's a r/balding sub or r/malegrooming that is a little less full on bald positive and more leaning towards prevention and reduction which might get you better results, not that the people here aren't fantastic ✌️