That's weird. I've never heard of a cat doing anything like that before. He just suddenly started doing this?

It can happen that even if a male cat is neutered he can still have the instinct to try and mate. It could be that if the measuring tape is his favorite thing than he becomes happy and excited when he has it and for him the way to satisfy the feeling of being happy is to hump something. At my house if someone makes a bed but leaves a lump in the comforter, one of my male neutered cats will hump the lump, he has nearly stopped because we make him stop when we see it. But i would still just recommed asking your vet what he/she thinks next time you go. Good Luck!