with welts- can happen in any viral fevers. Have him seen by a doctor.
For fever, she may give Acetaminophen
syrup in a dose of 15mg/ kg/ dose repeated every 6 hrs for 3 days. Give him plenty of fluids.
For welts, she may give Syrup Cetirizine
(5mg/ 5ml) 2.5 ml twice a day for three days.
If excessively itchy, lotions like caladryl
can be applied on the skin.
If fever persists /oral intake is poor/ he is dull/ develops breathing problems- he must be taken to a Pediatrician
Some infections like throat infection/ urinary infection can cause welts.
Hope my answer helps you.
Wish your grandson a speedy recovery.