Hi, I am Dr. Peter and would like to help.
1- How long has he been drinking more than usual?
2- When did he start having problems with his back legs?
3- Is he losing weight?
When a dog drinks more than normal it is a sign of either an infection, kidney or liver disease, diabetes and even cancer. The back leg problems could be the result of weakness by any of the above condition. Or, a separate problems like arthritis of the hips or vertebral column (back). I would recommend a hands on exam with your veterinarian; they will need to do some blood work and urinalysis.
If you have additional questions I will be glad to answer them.