I think that men are unconsiously effected negatively by things in their everyday life... things that unbeknownst to them results in a lower sex drive. I don't believe that I would be overly concerned... as long as he hasn't cut you off sexually completely and the rest of your relationship is going well. Are you noticing a difference in his overall behavior? Have you talked to him about it? Maybe he is doing this without realizing it... and it just needs to be brought to his attention so that he can reflect on his current situations and find out which one is resulting in the lower sex drive...

good luck