Hi Tiffany, I'm so sorry to hear this. I know how you're feeling regarding the Hip Dysplasia.
The reasoning behind keeping them active is to build up muscle around the joints. It also helps to keep them from becoming stiff. HD is at it's worst when it's cold and they're first waking up. Kinda like us as we age .
That's what I've been told at least. Just be careful not to overdo it.
HD is also very common in bullies. Most have it to some degree, but because of their anatomy, they don't normally show symptoms unless it's very severe.
Hopefully with the right meds and exercise, Rumble's symptoms won't be bad.
Keep us updated.