Almost as soon as we were married .
He is always looking for hard core stuff .
He was undoubtedly fantasizing about it already then. Wbu, was that when you started getting interested too?
Guys have a refractory period when they cum and then feel guilty about whatever it was they were thinking about when they cum. I know when I was married and fucking my ex-wife I used to shut my eyes and imagine her being bred by BBC just to get over the edge, I couldn't get over the line without it. But then I'd feel incredibly guilty about it and worry I'd lose her if I told her. She cucked me once (not with BBC) and eating the load out of her was the most exciting sexual experience in my life; when I replayed it internally to masturbate it was always a massive BBC inside her stretching her tiny pussy out. But when I eventually talked to her about making this happen she wasn't interested in making one of my "dirty little fantasies" come to life.
It genuinely sounds to me as if he is reaching this wall to cum and then feeling guilty about it and trying to exorcise it from his life. But because he can't erase what's in his head he uses it to come over and over before eventually looking for more material. I think you need to have a chat like I said before and impress on him that he isn't gonna lose you over it because you get turned on by the exactly the same thing. That what you have with him is emotionally rather than physically fulfilling; and that you don't like to see him unhappy because of his internalised guilt complex.
I'd suggest that one way you could help him get over this would be to get him a cock cage so he can get horny still but not cum. Ask him to buy a BBC dildo to use on you so that it is still him doing it. I have had Masters who explain the cock cage as necessary precisely because of this post-cum guilt complex; they would rather me not shoot my load and run away but keep me on edge desperate for more. Of course, how long you keep him locked up for is up to you, probably best not to overdo it at first. Basically you Femdom him, nothing overblown, but so that he can relax about his fantasies because he is not gonna lose you and you are forcing them to happen. Then it isn't "him" at fault, but both of you as a couple. Get him to articulate what he wants to be done to you while he is using your BBC dildo - and you articulate your own back to him, the filthier and more outlandish the better.
Believe me, I think to most of us cucks here he sounds like a lucky man to have a partner into the same thing as he is. You just need to stop him feeling guilty about it after he cums.
re: what IR sites I use. Just the usual - I torrent what I can anyway. I tend to prefer porn where the girls are obviously loving it rather than faking it for the camera. That is probably why I prefer stories more because then I can add my fantasies around the edges. But, when I have a few hours free nothing beats a butt plug, cock ring, my sounding kit and a bottle of poppers while edging myself for hours to black cock worship! Urethral fucking is hot but never gets me over the edge so I can go for ages!
PS. You can help him avoid this post-cum refractory period altogether, but it is quite hard-core and unless he expresses an interest in it I wouldn't push too far as most str8 guys have a panic attack if they do any sexual play they perceive as "gay." Basically the longer he goes without cumming, the more his prostrate will swell up. After about a week it will be very full and you can milk him anally - he'll get the endorphins of a cum but he wont lose his horniness after. But he needs to be relaxed about you fingering/dildoing/fisting him else he'll just seize up and his brain will be fighting whatever you are doing.