Thanks for writing your query.
Hives usually appear due to some allergic reaction
or some insect bite or due to drug allergy
You should get him examined and get the following investigations to know the cause of infection:-
1. Complete Blood Count
2. Absolute Eosinophil
3. Malaria and Typhoid test
4. Urine examination (Routine and Microscopic)
I hope this is helpful to you.