This is happening more and more now, and I'm not at all anti porn or the like, but I think the way it's so easily available, is problematic. I say this as someone who has worked in the adult industry, and despite things like this happening, there are folks who are able to watch porn without it getting to this level of addiction.
I recently listened to an audiobook called The Brain on Porn. It's an amazing book actually, and I think that it hits the nail on the head for a lot of people. It's unfortunately often responsible for diminishing relationships when someone develops habits that turn into addictions. If people access it from a younger age, it can lead to forming completely unrealistic ideals for how relationships should be. It's a fucking disaster.
His behaviour is very much one of a porn addict. Recovery is possible, but he probably doesn't even think he's got a problem.
He definitely has a problem.
It's not just the volume of porn that he has, it's the fact that he's kept downloading it, lying about it, downloaded it on significant dates etc. That requires active thought. Premeditation. It's not mindless thought.
Unfortunately there is no middle ground here. If he wishes to continue his relationship with you, he will need to permenantly delete collection. That's assuming you'd want to stay involved. If you do, and you'd be prepared to go through detox with him (this is an addiction and needs to be treated as such), it's not going to be an easy process. For example he might show you he's deleted files, by doing it in front of you, but what you might not realise is that he's probably got physical backups on external hard drives, disks etc. It requires an absolute hardcore removal of porn and the available channels it comes through.
He's pretty far gone given your descriptions, so if I'm honest, I'd run as far away as you can from him. If he's into cheating clips, there's a fair chance he's dying for you to do something like that involving him, there's a lot of cuckolds about.
If he's this far gone, he's probably talking to sexworkers too, either camgirls or phonegirls. He's likely aware that you're not at all into his fetishes, so if he wants someone to talk to about it, he will need to pay for it unless he's on fetish discussion sites. He likely is.
I've accidentally written a novel, but I'll stop now. Happy to help you make sense of anything if I can.