This is not something that I would even begin to give suggestions other than to highly recommend you take him to your vet.
There may be a number of problems going on including dehydration, diabetes, kidney disease or hyperthyroidism. Your cat really needs to go have a check up and will likely need to have a comprehensive physical and blood evaluation to start.
Good luck and hope that he has many years of quality life left.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street Units, 40-42
Tel. (905) 771-9855