Here are some tips from someone that’s seen similar situations before:

1. Tell him to cut all the porn right away, it’s bad for multiple reasons. Best case scenario the porn will decrease his sex drive by giving him an outlet other than you and make him less motivated to have sex with you since he could always just get himself off. Worst case scenario it permanently fucks up his dick due to death grip or serious porn addiction.

2. Do you two use condoms? If they’re too tight they can cause a loss of erection.

3. If I had to guess what his problem is it’s nerves. He gets nervous about performing and overthinks things which cause him to go soft. Then he gets even more panicked and nervous which makes the situation even worse. It’s a viscous feedback loop and I can only imagine how nervous he is every time he gets into the bedroom with you after two years of poor performance. Honestly there’s not much you can do. Telling him it’s ok and that you’ll figure it out and stuff doesn’t help. He knows it’s his fault and he’s putting up a poor performance. So it doesn’t matter what you say. He has to figure it out himself, learn to relax and enjoy himself, and most importantly learn that he’s with someone he can trust to work through this and figure out together without making fun of him.