My Son Had Two Boils Underneath His Armpit. Took Him

My son had two boils underneath his armpit. Took him to the ER. They poled holes them but only one drained fully. So now he is left with one hard bump even though after over a partial course of bactrim liquid antibiotic it lools as though there is no inflammation or pain anymore. Should i yale any further steps or just keep an eye on it?