The Purina formulas are high enough protein to prevent muscle loss, aren't they? Adding green veggies also lowers the protein.
What is the canned food? Some are high carb and high fat compared to others.
How old is the one on a diet? Is he cleaning up after other pets or kids and getting extra calories that way? He should not be fat on those calories, unless the canned is dished out with big, heaping spoons and he is getting Kongs full of treats or peanut butter.