Well I havnt actually read, nor have any knowlege of the parelli hill theropy theory. But..... I think hills are a expellent thing to incorporate into training.
My two horses live on hills. One lot of land I lease is all hills, and they are pretty steep in many parts. I believe it gove them quite a work out sometimes. And I think they actually enjoy them, they love to hoon down them when they are all excited....much to me going eek some of the times, lol.
But yea I think they are benificial to training. When I come across hills out riding I always get my horses to go up them, sometimes at a walk, sometimes trot or faster, depending on how they are going that day. I think it works on there breathing and over all fitness also, not just on the hindquarters, so it quite effective.
I don't know about the whole joint thing, my boys seem no worse for wear, and they are aged, 18 and 20 yrs old. So I think your 8 year old fella would be fine.