My 5 yo son has had high fever of 103f for two days and it doesn't go down with medicine. results from urine test negative. what should we do?

Depends on situation: If you have had a urine test done i assume you also had an evaluation with the testing. If the doc didn't find an issue that needed antibiotics you need to wait a bit longer & see if it resolves on it's own.Many viral illnesses act this way & often produce a distinctive rash at the end of the illness. Persisting fever is the bodies healthy effort to end the illness & is not necessarily bad.

Febrile illness: You don't tell us what medicine you are giving your child.At this point we have to assume your child has an viral illnessagain you mention no symptoms other than fever. So i would push fluids, give Ibuprofen every 6 hours for temp over 101 and put him in a tepid bath when you see temp going higher.Fever may last several days.