Thanks for placing your query through HCM.
I can appreciate your concerns.
It is very likely that he caught this virus from pool.
Viral infections are very common in young kids & it is not uncommon or unusual to have 10-12 infections in a year even for otherwise normal & healthy kids.
Can use paracetamol
syrup in appropriate doses & ensure he is drinking enough liquids.
There is no reason to worry.
Hope I have answered your query
Wish you son good health.