It's quite possible that it's not him but a family member who has a disability but the placard goes with the person with the disability, not the vehicle. Many people just leave it in the vehicle and other family members take advantage of it, which is wrong. They could be fined or have their placard revoked for doing this but I've never seen that happen.

I suppose you could have called the police and they would have had to show their person with a disability card to prove the placard is for them personally but mostly the cops can't be bothered to send someone.

As for you trying to determine a disability with your eyes, it doesn't work that way.