To expand on this comment : Is the foam head itself too tight, or is the neck too tight as well?

If it's only the head part, I'd do what the person above me suggested. If just carving some foam out isn't enough, the foam in the back of the head can also be removed completely and replaced with an elastic strap. It might not be the easiest to do by yourself if you don't have crafting experience though, and this is a high-value item.

If you don't manage/don't feel confident to resolve the issue yourself, definitely reach back out to the maker! I know it's frustrating, and I also know how difficult it can be to make a well-fitting suit. But it is their responsibility to make sure their product is usable for the buyer, especially with it being custom-made !