How can i acces my whistle register ALWAYS?

Okay, so i'm a 15 year-old baritone, my vocal fry ranges from E2-G2, modal voice from A2-Bb4, falsetto from B3 to D6 and sometimes E6, and my whistle register goes from A5-B6. So the thing is i can acces my whistle register, and i can go up to said notes with it., (and im not sure if its whistle, cause wikipedia says men dont have whistle, but if it isnt whistle , thenwhat is it?cause it sounds like a whistle, and feels VERY light and unforced, unlike the highest part of my voice which is a little straining C6-D6) And the thing is i CAN acces it sometimes, not always. Its hard to access, but its there, i mean its so hard to acces i can be using it going to B6 one second and the next one i try to do it again and it just doesnt happen. I try different mechanisms, like changing my mouth shape or opening or closing my throat more. But nothing. It doesnt seem to be under my control yet. It just happens sometimes. I mean, i can be all day trying to do it and i can only get it about 5-6 times. And like i said , i can be able to use it like in one moment and i breathe and a second later i cant acces it . Why? What can i do to access it always? I've seen many men do this like Brett Manning, and Christina Aguilera does it up to C#7. Help me please