I would certainly make him pay his way. If he travelled the world for 18 months then he must have bought food, paid for transport and accommodation etc. so can't expect a free ride now. (If he does, he should expect it from his parents, not from you.) My daughter volunteers and earns very little otherwise but pays for the food shopping for the three of us 2 weeks a month which seems equitable to us. He's getting a comfortable home with food, heating etc thrown in (and probably cooking and washing done for him) so at least 20% of earnings seems fair to me.
If he claims to be saving for a deposit etc, you could suggest he does that through you so he's not tempted to spend it on other things.
Bought the wrong Hot Cross Buns 😩
Water Pollution -“ A National Disgrace”? A case for renationalisation?