Sores inside mouth and under chin

Hi All, I have taken my cat to the vet 3 times, each time he has been given antibiotics and this almost completely disappears only to flare up even worse. In addition to what you can see in the photo, he also has sores in his mouth. Now they want to do about £300 worth of tests and then potentially remove his teeth. I really dont want this for my cat, its like de-clawing and I think its cruel so I thought I would do my own research to see if anyone has any ideas and if there is something I can do for him before he goes for his biopsy. He seems pretty happy despite this and is eating fine.

Here is a pic