What am I doing wrong?!

My boyfriend used to be just fine as far as sex drive goes. He was always a little unsure about it because he was super sheltered even through high school so he didn't know what the hell he was doing, but he always seemed to enjoy it. But recently he has been getting soft in the middle of sex and his sex drive is super low. I try to change it up every once in awhile and try to find new things he likes sometimes when he seems extremely uninterested. Am I doing something terribly wrong? I meant I gained a few pounds but I'm still pretty fit and active because I'm a firefighter... I just don't know what to do any more because it's kinda taking a toll on us even if he doesn't say it out right. Suggestions? I'm pretty open minded.