Is he prone to upper respiratory infections, which can affect the eyes too? Allergies? Are his third eyelids slightly visible? It kind of looks like it to me. That could mean some sort of an illness; it could mean he injured his eye somehow, although I think the 3rd eyelid is visible in both eyes.
How is he eating/drinking/acting?
I suspect if you send those pics to your vet, they may want to take a closer look at it in their office. But you can try and see what they say.
Hello, he is acting, eating, and drinking fine. I went back to old photos and his third eyelid’s been showing since months ago, and the vet never pointed it out. I will ask the vet specifically about it, though. Thank you!!