Welcome to HCM.
Probably he must have got ear infection
which is very common infection that one gets from a swimming pool. He may have swallowed some debris from the swimming pool.
Since he is otherwise healthy and not drowsy, you don't have to worry.
If the child becomes drowsy or has high fever
, then you have to immediately take him to ER otherwise you do not have to be concerned. Do as advised by your doctor.
Wishing your child speedy recovery.