WTF Someone is shaving my cat! What should I do?

So the other day I was petting my cat and discovered a patch of skin on his side that looked like it had been shaved. It was stubbly. Then I noticed his belly looked and felt partially shaved as well. Today, it looks like someone shaved even more of the belly nearly down to the skin. Almost his entire underside is stubbly and nearly bare. The rest of his coat is intact.

I can’t imagine any medical condition could cause his hair to become suddenly stubbly.

I’m assuming someone’s shaved him, and am not sure how to find out who is doing it. He roams around the neighborhood, is very friendly, and naturally loves people so it could be a few different people around here. I’m keeping him indoors for awhile until I can figure out what to do. He hates being indoors, and I don’t want to restrict his freedom permanently because of an asshat with a buzzer, but I’d rather him be indoors than violated by a freak.

Any advice on how to handle this / what measures to take to put an end to it? Anyone ever have to deal with this problem?