...and one thing I notice is that the departure from chronological order bothered me. "Wait, he's not supposed to be that old yet..." Did that bother anyone else?
I think your best bet would be to decide how you want to frame the story - are you approaching Alex via the mystical side of his life, or are you taking a more physical approach?
If the story is about the mystical, Son of Zeus side of Alex, then the OOB scene would likely make for a good prologue.
If you're going more with the historical, physical-type approach to Alex, then something like the blooding is probably a better choice for the prologue.
...that the beginning of the book is an indication and a promise re what the rest of the book will be like, yes.
Thinking about this one, because both aspects will be there, but I'm expecting it will lean towards the physical.
To my mind, mystical does not equal fantasy/not historical, but I know not everyone else looks at it that way.
Thanks for the input.