Sexless marriage... should I worry about him cheating?

We have been together for 10 years, married for 9. We have a blended family with several kids of all ages. We have had our share of fights, ups and downs, and discussed breaking up over the years mostly instigated by me. But I never pulled the trigger because of lack of money and instability. So now for the past year, his "libido" has been very low and for the recent six months almost non-existent. I worry that he is not attracted to me, which he denies. He does not show the typical signs of infidelity. Super secretive, out a lot, improving himself physically, etc., But I can't help but think that having sex once every 5 months or less is ok with him. He takes meds which interfere with his ability to perform sometimes and he complains that he hates to start and disappoint me but he doesn't even try. I am completely shut down and out. Sex has never been high on my priority list but at this point I am sad, feel rejected, a little mad that he refuses to ask a doctor for help, and scared maybe he is going elsewhere. Any ideas? Serious replies only please, thx.