How do I get rid of my sons seborrhea?

Try a shampoo... ...With either tar, salicylic acid, zinc pyrithione, or selenium sulfide. (if your son is very young, take care not to allow the shampoo to get in his eyes.) if none of those are strong enough you'll have to visit a dermatologist to get a prescription-strength shampoo, plus perhaps a steroid foam or lotion.