
Hi Fiona,
I can only relate to my old GSD that I used to have. We got him when he was 18 months and I didn't like the look of his gait, or the way he used to squeal after too much excercise. He was diagnosed with HD and put on pain killers. We were also told to keep him lean and trim because too much weight obviously exacerbates the condition.
He was in quite a lot of pain when he was younger despite all of the usual precautions, BUT, he did seem to improve on the pain front as he got older. We used to give vitamin C when he was particularly bad as I was told by the vet that this can reduce inflammation in the joints. Also, I seem to remember that we were told not to make his diet too high in calcium and protein as it also exacerbates the condition. It is vital that you control rapid growth, seeing as he is so young as this also doesn't help.
They may offer you a surgical solution if his pain is this bad, but you must weigh up all the pros and cons before going ahead and arm yourself with as much information about this debilitating desease as possible. My old boy ended up with a cart on wheels for his back legs as he got older as this was our only option for his mobility. But he lived a happy life and died of kidney failure at a good age.
Good Luck, I hope this helps.