I am worried about his looking skinny; weight loss can indicate many problems in older dogs such as liver or kidney disease, chronic pain, cancer, and heart disease. A good physical exam would be a good place to start.
If he is dripping urine, that can be from a bladder infecton. Pain in his bladder might cause him to bite at his back. Urine dripping can also be from prostate problems, so I would have his prostate checked (part of a complete physical exam).
Another possibility is back pain. If his back hurts, he might bite at it, and if the nerves to the bladder from the spine are affected, that could cause him to drip urine.
So the place to start is a good examination: listen to heart and lungs, palpate lymph nodes and internal organs and prostate, check reflexes, palpate all over for signs of muscle tension or pain.
Let me know what else I can help with or answer.