Hi and welcome to HCM.
It sounds like inectious disease so I dont think it is related to his previous medical history. It can also be allergy
reaction. WHatever it is,he should go to a doctor and do basic blood and urine tests.Usually this is respiratoy or urinary infection but some insect bites
may be considered as well. Till you visit doctro, keep him hydrated and reduce fever
if it occurs. Wish you good health.