Honestly, I would say that if he doesn't respond to you when you ask him politely then tell him in a not so nice way. If he tries to come over to your space sick, refuse to let him in and tell him to screw off.

I am like this and people get the hint real quick to leave me the hell alone when I'm sick. I don't get sick that often, but when I do I usually am stuck in bed for a few days or more. I have a difficult time controlling my mental health issues when all of my energy is being directed towards healing my body. People learned real quick to leave me alone because I become extremely rude and hostile towards people when I'm sick. Picture me at my worst on here when I fight and argue and multiply that by 5-10 and you get the idea.

So yeah, my point is sometimes you have to assert yourself to make people respect you. Friends/relatives outright fear me when I'm sick because I have developed such a reputation of hostility when I'm sick.