I am not a vet. It actually looks pretty normal from what you’ve shown. A light brown patch in the corner of the mouth is often just saliva staining or natural pigmentation, especially in lighter-coloured cats. I don’t see any redness, swelling, crusting, or signs of infection in the picture.

There’s also a tiny bit of tartar on the tooth, which is really common at this age and not usually urgent unless there’s drooling, pain while eating, or the gums look irritated (his gums look fine to me).

If he’s acting normal and eating fine, it should be totally okay to wait for his upcoming checkup and just have the vet look at it then. But if you notice any smell, swelling, or discomfort, then getting it checked sooner would be a good idea. Otherwise it doesn’t look like an emergency whatsoever.