talk to him, share how you feel. but careful not to make him feel like he is letting you down because he doesnt feel up to having sex quite as often.
I had the same problem with my partner. it was difficult to broach the subject without it causing arguments. In the end I found it easier to just buy a really good sex toy like this one:
http://www.lacoquette.com/playtime/delight.html
until he felt up to it again!