Hope he is not on nappies anymore.
If he is off nappies, its more likely to be a fungal infection
Though he was given 3 anti-fungal creams, if fungal infection is still suspected, a swab can be taken from that area and tested both for fungus and bacteria.
Depending on the result, he can be put on oral medications.
The local area has to be kept clean and dry.
Try to keep it open as much as possible. When needed a soft cotton underwear will do. No nappies should be used.
After stools, gently mop it off with wet tissue. Do not wipe it hard.
You may apply Zinc based barrier creams like Siloderm thrice a day for a week.
In the groin, instead of creams, you may apply canesten
Its wise to rule out worm infestations that present with peri-anal itching and excoriations.
can be tried.
And oral zinc for about 2 to 3 weeks will help.
If nothing works, you must visit a dermatologist- who will guide you further.
Hope my answer helps you.
Wish your son good health.