@mcahill,
You need to get him back to his doctor; this is probably a pretty common side effect of his cancer treatments.
It may be something that Viagra or the like could help with, at least to assure that he could get an erection as that might be a fear on his part, that he can't get to that point so he's not even trying.
And, while at his doctor's ask for a referral to a therapist. You and I both know that sex and sexuality isn't just in the naughty bits; it's also in the mind.
So I suspect this is a two-step process. 1) To handle mechanic-type issues, make sure the parts are working or can be helped along if necessary and 2) work on the mindset. And work on this together, and frame it as a mutual interest and a mutual solution and a mutual thing for you both to work on together, rather than his problem alone.