Peggy, My Beau had the exact same abscess due to a bite when he was only one, the locum vet (in Slough in those days) initially missed the diagnosis as his symptoms were partial paralysis of the back legs that this young vet had decided were due to a spinal injury from a fall or car strike. It was only when I pointed out to the vet that I'd noticed a damp patch on his back and neck a few days earlier that he checked and found the lump, then noticed his temperature was raging.
He squeezed the abscess there and then, a plug of hair shot out followed by a high pressure stream of hot green puss that hit the vet squarely in the face
I was holding Beau's front paws at the time and he was in such pain he bit right through my finger, poor little man, landing me in Wexham Park hospital with a swollen infected finger getting a Tetanus shot.
Anyway, it turned out the pressure caused by this huge abscess had pressed on the spinal cord and trapped a nerve causing the paralysis. Once drained the nerve was released and he regained full use of his back legs, a course of antibiotics brought his temperature back down quickly and he healed perfectly. He's 13 now, and still thriving despite a number of rubbish-vet diagnoses over the years.
It sounds like you caught Oscar's abscess before it got this bad, and if it heals the way Beau's did, Oscar'll be back to normal in no time. Don't worry about the lump this soon, it should continue to diminish until it has completely gone.
As for his local grazing, you can get "Do Not Feed" collars on-line, or if you know where he's likely to be getting fed, have a word with them and let them know he's under the vet for his weight.
