From my own experience, my Sky was neutered later in life and he has a very large broad head. My cat Starr was neutered at around 5 months and he has always had a very small head but it went with his small frame. Now Starr's head looks way out of proportion because he's gained a lot of weight due to the pred that he has to have for the rest of his life. His feet are also very small and they don't go with his huge body at all.

My RB Pepper was neutered before I adopted him at 9 weeks of age but his head was normal. He grew into a large stocky cat and his head was always in proportion to his body.

Do you have any pictures of your boy?