Suggest Medication For Infected Blister From Sand Flea Bite

When my son (aged 1) get bitten by a sand flea, it usually ends up as a normal red bump. But the following day it turns out to be a huge puss blister and i don't really know what to put on it and it really irritates him. I would appreiciate any advise as he seems to be suffering from this problem and doctors just keep giving him antibiotics for it. It is costly to keep taking him to the doctor which is atleast twice a month that he gets it.