I always thought it was hormone related but maybe its just age. He is 22 years old. I know my head resembles his with e very passing year.
Hahahaha! Hilarity!
At 22 years, he's still relatively young. So I hesitate to put it on age. There was a time that my Bixby had thinning issues as well. A bit on his head, and far more heavily on his chest and back. Turned out to be Mojo Molt. And that lasted about a good 6 months before filling in, too.
Only thing is, it was a one-time thing. And as I understand it, most mojo molts are. I believe I've heard of a case when it happened twice. But a somewhat regular recurrence over the course of years? Not saying it can't be Mojo Molt, but regular recurrences must be exceedingly rare.
As for your female ekkie not having the condition, that would match up with Mojo Molt as well. Almost all recorded cases of Mojo Molt happened with male eclectus parrots.
Personally, I'd take him in to see an avian vet. Just to be sure. Although sadly, many AV's don't even know about Mojo Molt. I suppose due to its relative rarity, the fact it isn't treatable anyway (short of waiting it out), and that it's more or less an aesthetics issue without any adverse effects on general health.
Of course, given that the effect is so localized, it could simply be a situation where the feathers just happen to be rather thin in those 2 areas. So whereas the molting would otherwise be barely noticeable, in those 2 specific regions there aren't enough feathers to cover the process. Not much different from healthy kids who start going gray in high school, or whose hair starts to recede very early on.