I've never used Siri while out and about, I still use my TomTom It is plausible he searched for massage and got a brothel. The fact the history goes into the profiles of the prostitutes though makes me think he did have a squiz.
It's a hard one. On one hand he sounds like a really decent trust worthy guy. On the other, from personal experience men that do this sort of thing will always deny, then gas light and make you feel like you don't trust them/are crazy then just become more careful in the future. I would say give him the benefit of the doubt at this stage although I suspect even if Siri took him to that page he navigated within it.