Poor Angus I am so sorry to hear that he is hurting.
My GSD Penny suffered like that too, she used to love lying upstairs on the landing and I used to have to help her come down and as it went on had to stop her going up in the first place. I just knew about the oil supplements then, this was a few years ago, but I think products for dog health and wellbeing have come a long way since then, in the supplements area especially. I know I would be tempted to try things like Pawws if Penny were with me now. The only thing that bothers me is that there isn`t any ingredients listed, but thats just me, I like to find out what is in stuff whether for human or animal use.

I did look on the pet symptom solver place to see what advice they had Difficulty getting up, what you can do for your pet and scolling down towards the bottom they had some advice on asprin and supplements that may be useful.
I know there is a lot of advice out there and before my computer crash I had most of them bookmarked! I am sorry I can`t be of more help, but be sure if I come across anything that I think may help my hero Angus I will be sure to let you know. Please give him a big hug from me and Bob.