Hi Timmy, You say you have caught him 3 times watching porn, so just how much do you think he views porn then, the problem with porn it can switch off his brain to sex, its not that he does not find you sexy anymore, its his brain its gone into a sexual overload.
Now you need to talk to him about it, one thing when he's out would be to install a parental lock for porn, see if you can fund where he has his porn sites, and get them cleared out, along with any porn downloads, that's unless he has them locked away in the computer, you could also look at the computer history, to see were he has been going to, that's if you want to go that way.
But he does have away out if he wants it, if he wants to get him to PM me, and I'll tell him what he needs to do, so you can both have a sex life back.
Good Luck
Try giving him oral that always works. And he could do the same for you.
You should talk to him. Maybe he is just sleep-deprived. Maybe he could take some supplements like L-arginine and start working out. That should rev him up a bit. But it could just be lack of sleep.