My dog has a mass with a white spot in his mouth?

So I had noticed that my dog seemed a bit off lately. Sleeping a lot, eating his kibble much slower. He's still eating but seemed uncomfortable.

I examined his mouth and saw this (see pics). I called the vet and they can only see him the 27th of July. They didn't seem to worried considering he's still eating/drinking. What do you guys think? Any advice is appreciated.

He's a mixed german Shepard +-1 year old. His history is unknown as he's a rescue dog but he didn't get teeth extractions before I got him.