Mouth injury/rot

[Minkness]

Soooo, my new rescue leo has either an injury/infection, or mouth rot.

I have never dealt with this before and just need a bit of guidance. (No pics yet, he's very squirmy). It looks like a dmall swelling that pushes out a bit and when he bit a tissue (i was trying to see if it was bleeding or pussy) it was indeed bleeding. There's no real discoloration, just swelling, what looks like a separation between the lip and gum, and some blood.

He has not been eating and he's in a steril tank with a hide, hot spot, cool spot, and large, full water dish.

I cleaned it out last night with a bit of listerine on a cotton swab just to be a bit proactive, but I'm sure that's not the beat way to go about this.

So...help?