Sudden lumps under my cat’s skin

About two weeks ago my 14 year old cat developed what seem like fatty lumps under his skin, 6 on his back and 1 each on his shoulder and hip. Initially I didn’t think much about them because he’s had a small one on his back for over a year that the vet said was just acne, but the new ones are bigger and growing very quickly.

I made a vet appointment for a month from now but now I’m second-guessing if that’s soon enough. I don’t think they’re cancerous since he’s eating well, hasn’t lost any weight, and seems blissfully unaware that he even has them. But they’re unmistakable when you pet him, the biggest one (in the pic) is already about as big around as a quarter and is raised up about 1/4” high and is starting to feel a little crusty.