“Are you getting these in ?” when walking to the bar, is the easiest thing to say.
If he says “no I can’t I haven’t any money on me”, I would say “OK, I’ll get these”. But if it happened again, I would say “but you said that last time” and would buy my own.
The chances are though, that he’ll say “sure, what are you having”.
Don’t be shy otherwise some people will take the piss. Although it’s always better/nicer to be generous than mean and tight-fisted like Derek Starship, so either way you win and he loses.