He's probably just molting. If he's under a year old, then it's about that time for him.
If he had a completely green head, and now it just looks like he's losing those green feathers, they will grow back. Did you get him before he had feathers? If so, then he should look similar to how he looked when he first grew feathers, before they turned green.
I have a drake that is molting right now and he looks that way too. It's been about a month and his green is just starting to grow in again. Some stay this way for most of the winter though, and won't grow their "pretty" feathers until mating season in the spring.