If you are worried, suggest you to call matron instead of housemaster.
Don't worry, if you son is sick, I believe the school will take good care of him and and keep you updated about his situation.
My son was sick two weeks ago. The School informed us (not my son!) he was sent to the sanitorium for a night and he was back to school thereafter. I think your son's school will do the same.
If you are worried, suggest you to call matron instead of housemaster.
Thank you very much for your reply and suggestion. Not every school has the same practice though. My friend got notice from school after her daughter got sick and skipped school for few days.
I will try get hold of my son again today to clarify his situation before deciding whether to call the matron.
I guess he is only a bit 吾舒服 and 咋嬌.
Btw, my friend told me these two days the temperature dropped to 8 degree suddenly in UK, so many students got sick!