Does he seem to drink water constantly and cannot go anywhere without water? If so, it could be
diabetes insipidus, which is VERY different than
diabetes mellitis (Type 1 or Type 2).
If not, then it could be a number of things, such as an enlarged prostrate or something else impinging on the bladder and creating a sensation of needing to go, even if the bladder is empty. It could be a problem with the nerves or muscles that connect to and control the bladder.