Husband low sex drive

Hubby and I have been together for 6 years, married for 2. The past year sex has been few and far between. When I confront my husband on why he claims he has no sex drive and the thrill is gone. I know it bothers him since he did bring it up to the doctor and the doctor did give him Levitra but it doesn't help his sex drive. Is there something natural he can take? We both want to try to conceive but it's hard when I'm the only one who has a sex drive.