Thanks for your query,
as mentioned by you your son is investigated and all tests were normal so nothing to worry,swollen gums
is also mostly a part of ongoing disease process only and is also viral in nature.
Continue with paracetamol
till fever subsides,give him plenty of fluids,fruit juices as vitamin C
helps in healing.avoid all kind of fried and junk food.If his symptoms do ot get better than again take pediatric consultation.
Your son will be fine,