Welcome to FDMB!
I suspect that you transitioned Henry to canned food a bit too quickly. Many cats end up with some GI distress if you make an abrupt switch from dry to canned food. While it will pass, you may want to help the passage along a bit more quickly. I would check around at health food stores or even Whole Foods to see if any place has a particular probiotic, S. boulardii, in stock. If not, you can find Jarrow's brand on Amazon. Start with 1/4 of a capsule mixed into food. It things aren't moving in the right direction, increase it to 1/2 a capsule. Some cats will benefit from pureed pumpkin as an anti-diarrhea solution. Just make sure you get plain pumpkin, not pumpkin pie filling.
How is Henry behaving differently?
Some of the weight loss is due to the diabetes. With uncontrolled diabetes, when your cat eats, the end product of metabolism (i.e., glucose), doesn't get into the cells. As a result, your cat is hungry all the time but because the glucose is floating around in his blood, he's starving. insulin is what moves glucose into the cells and without enough insulin, the glucose isn't going where it's supposed to go. Since he's not getting the benefit of nourishment, diabetics lose weight. However, you're also correct in that a low carb diet its likely to cause some weight lose. The good thing about a low carb diet, though, is that cats are obligate carnivores which means that their bodies aren't able to process carbs.
