I don't mind that he's sick and in the office, but I'd greatly prefer
he not touch things on my desk while he's sick.
It's not just your desk.
It's the table in the conference room, the door knobs, the whiteboard markers, the handle of the coffee maker, etc. Plus you may not catch it from him directly, but he could infect another coworker, who infects another coworker, who eventually infects you a couple of weeks later.
Tell him the truth. That he shouldn't be coming to work when he's super contagious and spraying fluids all over the place (unless he wants everyone else to get sick too). And that if he does come to work, he should at least try to keep the contamination to a bare minimum.
Are you afraid of offending him? I guarantee you. You won't be offending him.
Some people are proud when others notice that they came to work despite being sick. Those people have the hero complex. Other people are also waiting for someone to tell them to go home before they'll take the next day off because they want to make sure no one thinks that they're staying home slacking off. And others still are just oblivious. And even in that case, I really doubt that you would offend such an oblivious person by telling him the truth.
Get some disinfecting wipes and some spray for your desk. Get yourself a flu shot (like I said, you may not catch it directly from him, the entire office may catch it before you catch it yourself).
And get your boss some of those disinfecting products too (and some would say that he should be wearing a mask as well, but obviously you're having such a hard time confronting him already, I won't tell you to go that far). But ideally, he should be the one washing his hands with antibacterial soap everytime he touches his nose and he should be the one wiping down objects everytime he touches something. But in case he doesn't, at least you'll have your spray and your wipes.
