Well first off, don't EVER try to "help" him shed any part of his body by pulling, scratching, or doing anything else of the skin. By pulling it off too early you can cause a horrible wound because the new skin underneath may not ready yet, and you'll rip it off too. This is a horrible situation when it happens, but it's easy to avoid, just never, ever try to pull any shed off.
Some areas do take longer to shed than others, usually the tail is what takes the longest time to shed, simply because it's got the thickest skin. However, every dragon is different, and there is no "regular" time period that a Dragon should shed in, nor is there a "regular" pattern that they ever shed in. So don't worry that his head is taking longer than the rest of his body did, that's completely normal.
Sometimes you can help by letting them soak in a warm
bath for about 15 minutes, then pat them dry, and apply organic, pure, unfiltered coconut oil directly on the shed that remains. I would only
bath him once or twice a week though, and obviously you're going to have to keep wetting the area on the top of his head, as it won't be in the water. But you can apply the coconut oil twice a day until the remaining shed comes off.