what time would you need him up by to be out of the house by 8.45?
maybe start waking him at that time and then work out what BT that would lead to based on his average night length. Might take him a little while to shift but he should get there.
Is his room light at BT? Do you have blackout blinds?
I try to draw all the curtains upstairs at bath and BT, to dim the light down a bit and get them (esp DD2) and their bodies used to the idea that it will be BT soon (and to maybe(?) help melatonin production etc...). Then DD2's bedroom is pretty dark. Some light still creeps in around the edges but it's dark enough that she is still nodding off fine.
It seems that the lightness doesn't bother your DS in the morning?