[X] Suggest a romantic gesture, something to see if there's a spark without crossing his boundaries. Saunter up to him, say something very suave and confident and charming, then give him a kiss. Kisses should be safe for him, right? Probably. You'll ask to be safe.
[x] Tell Issachar, politely yet frankly, that his lack of sex drive really isn't the most pressing issue. He's still hiding things, between his ambiguous mortality and his mysterious past and mysterious associates and mysterious powers. Ask him to please just explain, once and for all.
[X] Tell Issachar it's kind of dumb that he thought this would ruin the chances of things working between you. One of the many benefits of polyamory is that he wouldn't need to feel that pressure for sex. There'd be enough willing partners to go around, leaving him free to focus on the romantic side.
[X] Offer to let Issachar think over what he wants for as long as he'd like. He seems to do well with time to think, and you're in no rush to try and intrude or pressure him. And fuck if you know how to help untangle this.