Is it me or is it him?

Ok so my boy friend and I have always been freaky in bed and it seemed as though we were doing it more then not. I am in no means fat or ugly lol so I KNOW its not my looks causing the issue. Yet it seems that recently he sort of lost mojo to get it on ( I'm the one initiating sex as well as being the one who wants it all the time as well as being the one who is truly into it. The biggest thing bothering me is he isn't get as big or as hard as he use too. To be completely honest its quite disappointing because how hard and big he use to get was so WONDERFUL. Whats going on? Is this something I am doing or is it something unrelated to me? How do I get him his mojo back? I am almost 99% sure it's not due to any other women...