My boy has cellulitis

Marvin has been limping for a week and over the past 3 days his leg has become more and more swollen and off colour. I got back from work today and it had doubled in size (which seems impossible as it was so swollen already) and it's black (not totally, but you know, bruised kind of look). The vet diagnosed Marvin with cellulitis, but said nothing can be done apart from to "sit back, wait and see". I'm pretty annoyed as he seems in pain. Anything that I can do..?

He needs pain relief and antibiotic..... he needs AB/s as this is basically an infection.
The vet is neglecting his case.

Never heard of this before ...

I actually had this myself 2 months ago and it can be very painful when it swells up.