|Signs of pain - narrowed eyes, tight face, stary ungroomed coat, ears lowered, tense body posture rather than relaxed posture. Difficult to see because of long hair. Attention inwards not outwards. Fewer movements of any kind. His head is lowish.
I'm a cat so I hide my pain. That's what we are programmed to do, to prevent larger predators seeing us as easy prey. We don't wimper or howl. We don't complain.
The only exception is when we might shriek in the middle of an violent attack or accident.
We suffer silently, uncomplaining and stoic, during illness or pain. So our humans don't recognise that we are suffering until it is really, really bad.
Probably the most obvious sign is lack of appetite. The litter tray may reveal bowel or urine differences. Our humans should take photos for the vet.
Your human can and should do something NOW. Get the free app - https://www.felinegrimacescale.com/phone-app
It will help them recognise our suffering.