I agree with Macabre; My mare does the same thing...well used to; she's gotten a LOT better, but it's taking a while, just because I don't think she's ever been taught HOW to use her body. I started out with some smaller hills, with less slope, and have been working from there...if she needs to go 'sideways' I will let her, and I just help her balance with my seat, and a bit with the reins (to steady her head a bit, and help her "lift" if she needs it)... The other day I took her down some stairs at a park; two weeks ago, she went down only a couple before she got really unsteady and I walked her off of them and proceded down the hill...this time she went all the way down; slowly, but she felt pretty comfortable under me, just a bit unsure. I try to get out 3-4 times a week though.
If you have any smaller slopes near home, I suggest you go out there, and work with him there for a while...he is young, and really does need to develop the muscles for downhill stuff. Unfortunately, once a week just probably isn't enough for him.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."