Op,
I've no idea how he access porn, but it might be through an internet browser, rather than an app, so you may not have seen anything.
I know this has to be ultimatum time even though I cant imagine leaving him over this, I know that the longer it goes on the more of a problem it will be between us.
I think you have hit the nail on the head here. You don't say how long it's been since you've DTD, but I suspect it's been a while. Can you see yourself spending the rest of your life in a (effectively) sexless relationship, whilst your DH gets his jollies from watching porn?
For instance, in my relationship, it was my XW that lost her sex drive. If we had sex four or five times a year, I was lucky, and when we did, there was no snogging or foreplay - she just did enough to get me ready, and then we had PIV sex. We really didn't have much in the way of intimacy - I felt she used me like a human dildo.
We talked about it, but she didn't want anything more. I did, and couldn't "go elsewhere" for sex (I'm guessing you wouldn't consider that either!), so we divorced.
You are a lot younger than I was when we divorced (I was nearer your OH age), you have a lot of life left to live, and there are a LOT of men out there..
I know men enjoy looking at porn Not all men, and not to the extent you say your DH is. I think he has a problem.
I just see it as he's replaced me with porn and maybe it's time you replaced him with someone that is willing to be actively involved in a mutually satisfying sex life.
Good for you on giving him more time. What are you going to do if he doesn't follow through?