Thanks for the query.
You might have developed cellulitis
It is deep bacterial infections.
There is possibility that some thing bit him while swimming.
This might have got infected.
Treatment is with antibiotics.
Please meet a dermatologist
for exact diagnosis.
If required even injectable antibiotics can be given.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.