
Sorry to hear about the problem your boy has. Unfortunately all drugs carry a risk and it's wrong of the vet not to give you the possible side effects especially as you asked if there were any. You have every right to feel annoyed and let down, your boy has reacted adversely to the drug and it is horrible to see him suffer when you were trying to help him :( You are doing the right thing not to give him anymore.
I imagine the vet prescribed this to ease the pain, but really you need to find out exactly what is causing the problem with his legs and causing the pain so that the problem can be treated.
If you have any glucose, you can add a spoon of this to his water which should help give him a bit of energy.
I see that you are off to see another vet and I would be asking to be referred to a specialist, and remember to write down everything you want to ask before you go and don't feel that you can't take up the vets time, this is what you are paying for.
Hope your boy makes a full and speedy recovery, let us know how you get on.