You need to go back to the doctor, if what they've prescribed isn't working. If he's stopped using what was prescribed he needs to go back to using it. Some people need to have lifelong daily maintenance measures for their eczema, not cure a flare up then stop caring for it.
If he's using any kind of perfumed soap or shower gel then stop. Try something plain with zero fragrance in the ingredients. It can take some experimenting with products to find what works.
Is he washing properly? Because if he's not completely clean that could make the itching worse. Has he been taught to get into all creases on his skin?
Some people with eczema find washing less often helps too. He's only 5, I think it's when people hit puberty they need to be washing daily/twice daily depending on what they've been doing.
Ask a pharmacist advice about how often he should be washing, what product to use, any cream they can recommend for after. Especially if skin is damaged because as skin heals it itches, that's a natural part of the healing process. The antihistamines will also help stop the itching (even if he doesn't have an allergy) allowing the skin to heal without him feeling the need to scratch it.
Keep his nails short and filed smooth with no sharp edges and put socks on his hands so he does less damage if he scratches himself in his sleep.