does he drull on his pillow? does he sweat at night? buy a pillow case cover this might help a little.

He says he doesn't drool or sweat. We use a pillow case cover, but it doesn't help at all. If he doesn't use the cover the pillow turns a sickly yellow color where his head sits

What does it smell like? Does his hair/head smell, too? Do the rest of the sheets smell? Does he eat a lot of onions, garlic, or any other foods that would come out of his pores? Have you tried switching his actual pillow? Maybe the smell is coming from the pillow itself or is reacting to his sweat or something. Has he tried changing his shampoo?

The sickly yellow color sounds like drool marks. Does he have bad breath? Maybe using a mouthwash before bed would help if it is drool?

He's drooling...

...unless the sheets smell like the pillow then I would say that it is his sweat that smells....

Best thing to do...don't wait to change the pillow case every 3 days...change his pillow case everyday and rotate his pillows...