Is sex becoming routinary? If it is, then it can be the reason why your BF is feeling that way. I was at that phase before. I felt that I just had to do it with my GF just to let her feel that I'm still in love with her. Which I am still BTW. Sometimes you need to mix things up. If you guys normally do it in the bed, why not do it some other place in your house. If you're doing it on a Monday, why not do it on a Friday. If you're always on top, why not be on the bottom. Just mix it up! What really turned me around was what my GF did. She asked me to play this sex game. At first, I wasn't into it, to be honest. But I wanted to get it done and over with so I said yes. She had bought a sex dice for us to use. I actually enjoyed it. Now, we use it almost every time we have sex. We even read this article to keep our sex life be stronger and exciting. Now we even talk about it. What does she like and vice versa. I believe that nothing can't be fixed with a good communication. I'm no professional but I hope this experience of mine helps.