What am I doing wrong?

I bathe my son every Sunday, & Wednesday. Well this week maybe starting Tuesday he started developing this oil residue on his right side of his head all the way to the back of his ear. It stinks. I thought maybe I wasn’t cleaning behind his ear enough so Wednesday & today I really payed attention to said spots but it’s not helping.. do I need to bathe him more, less. Maybe just shampoo his head/ behind his ears every day without taking him a full bath. Thank you

Edited to add this residue is sticky it makes his hair hard.