I'd try to catch him tonight when he's calm and eating.

Kitties have been known to teleport themselves to an alternate universe when they know they are going to the vet. Somehow they pick up on these things............

Also, when we neutered a stray, the vet advised to keep him in a quiet room for the weekend until he recovered. So, we put Seymour in the bathroom. He flew out of there and thru the barely open window in about 15 seconds! It took a while for him to trust us again, but he eventually came back around and did just fine.