About a year ago, my husband would've told you that we were in the same situation... in reverse. (I was the one with the new job/stress/fatigue)
It could be so many different things! It probably has a little to do with the stress and probably some physiological changes too. Throughout our lives, our bodies change (not just at puberty). It could be that his testosterone levels are off, or that something else is off. I agree with the others who suggested that he see a doctor. Have you confronted him about the problem? Could be physiological (hormones or otherwise) or psychological (stress).
When we were in this situation, I knew that I was tired, but my hormones were also out of whack AND my husband's attitude would totally turn me off. (He was a stay-at-home dad and didn't want to give me 10 minutes to myself after I came home from work) He'd get frustrated with me for not wanting to "hop to it" right away, - (I just needed to get dinner out of the way and take a bath) - then he'd get ugly with my daughter over minor things, which would turn me off and make me not want it at all.
Talk to him and see if you can figure out what the problem is. If it's not stress and there's nothing he can point to about your home life that is causing a problem, it may be his hormones.
EDITED TO ADD: Don't do like my husband and pout or get mad if he doesn't want to. Also, try to approach the subject in a manner that expresses concern, rather than saying "Why am I always the one that has to initiate it?" or "You're NEVER in-the-mood." Which is what I got!