I put both mine on adult asap, Lola was on adult around 6mths, Harry around 4mths.
90pounds is pretty big for a 10mth old, average 10mth GSD male is around the 70pound range. What does he look like? Does he have a nice trim waist, can you easily feel his ribs? Post some pics if you can.
He has pretty much gained his height now, although he may gain an extra 1cm by 15mths. If he were mine, and assuming he is not overweight (just large) I would attempt to slow his growth down by putting him on a middle of the road adult large breed kipple. He will still achieve his potential, just at a slower rate which may decrease his chances of joint problems.
If he is overweight, I would still put in on adult food and reduce his intake whilst increasing gentle exercise.