Stop beating yourself up.
You researched and are taking things into consideration. To do otherwise would be reckless.
You have made a decision and one that you didn't come to easily.
Whether it means anything or not I believe that his limping WAS a sign for you to help aid in your decision making process.
I hope the medication not only offers some relief from the tumor growth but relief for you as well in that you will feel as if you are doing everything in your power to help your guy.