How much dry food is he eating? Maybe he ate more dry lately and didn't feel hungry enough for all the canned.

One of my seniors never leaves a single kibble anywhere(Preacher). Sleet sometimes leaves her entire meal and Paxil is the fussiest eater in the whole yard. Her kibble must be soaked and there had better be a tasty treat in it somewhere or she won't eat at all! Sleet's weight is perfect, Preacher needs to lose a few pounds, but has other issues that interfer with that, Paxil needs to gain a few!

Duke's weight loss could be from the new medication. When Goldie went on Meloxicam and started feeling better, she dropped five pounds in a few months with no change in her diet. It just didnt' hurt to move anymore so she was more active during her day. Maybe the same thing is happening for Duke?