Boyfriend's sex drive gone

So as of the last year with my boyfriend, we have been experiencing some trouble in our Sex life. We are happy together and love spending time with each other. It's just as time goes on, he is less and less sexual with me. He isn't gay and doesn't have any interest in other women. It's very odd. We barely have sex twice a month and I'm wondering what it is I can do to help him? We have discussed low testosterone levels and a few other options. It has gotten to a point where he doesn't finish when we have sex because he's tired or gets too hot.... I've never had this issue in the past with partners so my automatic response is maybe I'm doing something wrong? Any advice on how to get him interested in playing around again? I've tried every fun, weird thing in the book and still nothing seems to get him in the mood. He loves to cuddle and and be close to me, but the intimacy and romance level is dropping. Please help!

Thanks