To me it sounds like there are some underlying personal issues for him that are rooted in his past, and not a problem in your relationship. Have you considered going to therapy together (or separately of course) to talk about these issues? I suggest together because that might make him feel more comfortable. Does he have other symptoms of depression? It worries me that he was just put on Zoloft and not offered any form of counselling or therapy, so maybe it would be good to get a second opinion.

I am again kind of going from personal experience. My partner has a pretty normal-high libido and I have a pretty low one. We haven't been together that long and I am hoping that I get better with practise... but sex makes me very awkward and usually I feel terrible afterward, which puts me off. It's such an emotional rollercoaster every time that the fun is kind of taken out of it! I find my partner very attractive but when we actually get around to sex I become very inhibited. That is nothing to do with him but with my previous experiences.

Also, porn is not necessarily a thing he is looking at for pleasure. As a shy person, it might be that he is simply embarrassed about thinking about it at all, but maybe he is sort of testing himself with it... it sounds like he needs a lot of support to establish himself in this. But the situation must be hard for you too, I will be thinking about you. Hope this helps a bit...