It's horrible stuff to apply! I made the mistake of carrying it in my bag once, the top came off, it was not pretty, went everywhere! Would a larger dressing work better than a plaster?
I get frequent things like that due to a condition I have, though different location, and have antibiotics, preferably to calm the infection down and stop it getting to bursting stage. Is there a walk in centre near you that you could go to or call the out of hour doctors to see if they would see him?
What antibiotics is he on? Could it be ones which aren't as affective for a skin infection if for a sinus one?