Bad breath and lip redness

So my cat is around 7 years old and male, and about a year ago we took him to the vet because his breath got really really bad. Needless to say we're not going to that vet's office again because all they told us was just to switch the brand of food we bought and also dismissed our worries about his cough, which we later found out was feline asthma and he was having asthma attacks.

We currently have him on a prescription to control his asthma but his breath situation has remained the same. Tonight I noticed the inside of his left lip was red so I lifted it up and noticed he had some sort of mass on the inside of his lip. I was thinking it might be an abcess but I'm not 100% sure, and if so are there any home treatments or do we need the vet to perform on it?

Thanks!

It's near to impossible to diagnose anything just from a picture, but particular when that something is a "mass".  That being said, I would take him in for a check-up, because any type of mass I would think would need to be examined, and if it IS an abscess, he probably will need antibiotics.

Hopefully it's nothing serious.