What is your schedule like? Is it more or less demanding than his?
I find that when I am less active during the day than my husband, I tend to have a much higher sex drive than he does, and have a higher expectation to have sex. (Our sex per week is about the same as yours, btw--and I think a lot of married couples would actually be grateful to have that high of a number, to put it in perspective... especially with young kids.)
Why shouldn't guys be allowed to have a lower sex drive? They're human, not horny sex-obsessed machines that are dying to jump your bones every day. (I mean, sometimes they are, but usually it's during the infatuation stage of a relationship.)
Just a thought.