Why are you encouraging him to be poly, when he (or you two) have so many problems around sex?
Why not just rebuild your own sexual relationship with him for a good long while, now that's he's gotten medical attention? Not saying you, yourself, can't date, but if his self esteem is that low (imagining you're not satisfied during foreplay?) it seems he can barely handle sex with his long time, hopefully trusted, partner, much less with a new person.
OTOH, if he's afraid he's not pleasing to you in bed, maybe you seeing other guys is also a blow to his seemingly shaky self esteem at this time.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 34), miss pixi's Dom for 3 years