How to I pay them back?

Hey everyone, not really a girly question but I wanted to ask advice on here and not ask my friends to sound “braggy” or whatever. Basically my boyfriend and I have been dating for 6 months, and for his dads birthday his brother paid for everyone in the family to go to Aruba this month. I’m so so grateful but I’ve been brought up to pay my own way, and it’s a little awkward for me also considering I’ve never met the brother whose paid my way. Anyways- how can I kindly say I want to pay him back without coming across as disrespectful.

Cheers