If he's received six weeks of antibiotics, then he almost certainly has an infection involving the plate/bone in his arm. It's very hard to eradicate infections with antibiotics alone when they involve foreign materials or objects like plates. If the plate has been in a while, we usually recommend removing the plate, treating with antibiotics, waiting, and then putting a new plate back in.
My guess is they're trying to see if they can get away with not removing the plate, and hoping the infection clears with antibiotics. It's possible, but not the ideal approach. If that doesn't work (and there's a good chance of that), they'll have to remove the plate, treat again, and put it back in later.