It looks like DM wet is 171 calories per tin, so 1 1/2 tins would be 257 calories.
"The average indoor cat should have 20 calories per pound to maintain weight. The average outdoor only cat should have 35 calories per pound to maintain weight. Indoor/outdoor cats are somewhere in between. o If your indoor cat weighs 10 pounds, then they should be eating approximately 200 calories per day."
If your kitty's ideal weight is 11 pounds then the can and a half should maintain his weight. Was he ever tested for hyperthyroid?