Not all fungus ointments... will work. So then another type of fungus ointment has to be used.
What may I ask, was the name of the antifungal ointment that you used?
There are also different kinds of 'fungus' infections/skin problems. ie: Ring worm, Yeast infections, and others.
And, there are specific 'fungus' creams to use for it. Either prescription or over the counter.
If it were a yeast infection rash... then for us, our Ped recommends one with "Clotrimazole" in it.
If he has Ring Worm... that is HARD to get rid of... and it takes time... this type of fungus... will be shaped in round circular rings/shapes and the inner area of the fungus will be red or scaly and patchy... AND fungus infections are contagious...and can spread....
Take him back to the Doctor... because his "fungus" problem is NOT going away....