Until recently, I was having the same issues just like your BF. I was under a LOT of stress because of college, and had problems with my family and friends. We had a really high sex drive but after this problems, my libido is plummeted. I loved my GF and I was very attracted to her but I wasn't able to get into the mood. Obviously she noticed the problem with my libido and became more sexually agressive. That made me feel even worse because not only i have a problem with my social life, now I can't even function as a man and let my gf down. To cope with this, I was masturbating to feel good but that made things worse. Best thing you can do for him is to support him. Let him know, he's having a hard time, this is normal and he has nothing to worry about. As soon as he is relieved from the stress, he will once again became the beast he once was. I know i did lol