There are wake up devices for the deaf that shake the bed. That would be one option.
Another would be to include in your Mom role, since this is new for him, a wake up, mmmm, nudge? Go into his room, put up the shades, turn on some music he likes (okay, Mozart can be pretty upbeat too!), and more ideas along that line.
You are Mom. As Mom, I believe you can help your children in many ways.
My Mom didn't turn me out of bed (neither did I have an alarm clock either) my DAD did. First was turning on the lights. Then putting up the shades. Then removing the bedsheets. Then physical (though gentle) removal from bed. :)
I don't think your son sleeps any different than any other 15 year old. You can turn him out of bed, but have a discussion about it in the afternoon (or on a Saturday would likely be best, so you can explore options if need be to get additional go gear).
He is a foster son, and I think he will remember your kindness in this area, and helping him learn to wake in the AM, as you said, less tense.
Oh, and an afterthought, do anything you do 40 minutes before you want him up. :)