I call villians river bet and he mucks, am I in my rights to ask to see his cards?

I did pay to see them, so to speak. I have been playing for many years and have never had anyone ask, so I assume he has a right to muck even though you called?

Reason I ask is I face off against a LAG player and I am trying to get a better feel of his range vs position and he mucks against me in this situation a couple times a session, info would be useful.