Hello and welcome to HCM,
G-6-PD does not present with rashes on the body.
As you have mentioned that your son has had an exposure to sun at the beach recently, the rash is most likely due the sun exposure.
The sun burn leads to red skin which later on turn itchy.
Your son is also having similar symptoms.
Topical application of emollient
can relieve the itching.
Avoid exposure to sun light.
Maintain hydration of your son as sun burns cause dehydration
Observe your son for a day or so.
If the symptoms do not get relieved by these remedies consult your pediatrician
Thanks and take care
Dr Shailja P Wahal