Well if he has a flat-shaped head since he was born, then the baby is not developing one, and my comments to that effect don't apply. However, this product looks to be a preventive rather than corrective measure, and a pillow with a hole in it isn't going to change things after the fact.
If you want to make one, though, I would just take one of those carseat liner things (I've seen McCalls and Simplicity patterns) and put extra padding in the top section and putting a small cutout where the back of the head would be should do it.
My apologies for the snarky comments. I just looked at the product, looked at what it was meant to help with (for the typical user - the one that carries the baby everywhere in the bucket so they start to get a flat head), and got kinda pissy. That happens when I see things made to correct the negative side-effects of not being held by a real person, without actually having to resort to picking the baby up

: Very glad to hear that that is not the case.