Husbands not that into it..

We've been together 3.5 years and I've notice his sex drive decrease as time goes on.. I'm 24 and he's almost 26.. I think we're to young to be only having sex on average twice a week. It's bc of him..not me...especially when we're ttc.. and I am the one pushing him into bed 99% of the time at any chance I can. Is this common or is it just him? I would just think a young guy would be wanting to "do it" more then that...