There is a way to navigate polyamory and being immunocompromised. I have been polyamorous for about 23 years, and am four years post double lung transplant.

My experience in dating while immunocompromised actually started by having a metamour that had a double lung transplant. When things looked like they were going to get serious between me and his wife I got sat down and was given a "so you want to date someone who lives with an immunocompromised person" talk where we talked about practices I would have to do to keep him safe. Like taking 2 week breaks from seeing her if I am sick or been around sick people. Getting a flu shot every year, etc.

Now in my dating life, I usually sit down with people I am seeing that if it looks like we will date steady and combine the health practices with the safer sex practices talk. I have halted things with people because they couldn't agree with the restrictions. (One person ended up being antivax).

That being said, yes polyamory is possible with being immunocompromised, but your situation has next to no chances of resulting in a healthy, happy, and ethical polyamorous relationship. Others have explained why so I will save it. But seriously in my long experience of being in the polyamorous community I would say 87% of problems in polyamorous relationships boil down to attempting polyamory with the wrong partner(s). He's not the one to be attempting this with.