Your last post seems a little contradictory, because he evidently used the internet on frequent occasions in order to set up your profile and read the responses.
I'm assuming you've got private browsing, so unsurprisingly, the internet history will tell you nothing. If his workplace has poor internet controls, it is also possible he views material when there. It's also not been unknown for people to secretly keep separate devices (tablets, phones) specifically for activities their partners wouldn't like.
The reasons I think porn is a feature is because cuckold fantasies have been commonplace in porn since before the internet, together with all the evidence that people with porn addictions that are kept from their partners, usually need more extreme scenarios in order to get the 'high' they achieved when they first started browsing. It is startlingly common for users to find that this affects their judgement of what is ethical or acceptable and this spills over into their real lives.
In situations like this I think you have to judge what's most likely, since at the moment he will not tell you, which is another term for lying.
Is it more likely that various behaviours and thought processes led up to this, or that he is an amazing man who'd never shown any signs of disrespect for boundaries before and had an inexplicable, totally isolated brainstorm?