I agree that a second opinion is always a good thing, but for the moment it's out of the question since the nearest specialist is about 2 hours away.

Bluesirtalis, I think I see what you mean in the pictures, but it's an illusion. The mouth is ok and an oral inspection was indeed done by the vet. (Thanks for paying close attention to the pictures, though!)

There was, maybe, a bit a swelling inside, way at the back, but even the vet wasn't sure.

I'll wait and see, as long as he doesn't go off food in a worrying way and the lump doesn't get bigger. If there's no change or the lump gets bigger, I'll try the anti-inflammatory drug, and then, we'll see...