I have been using Iam's weight control cat food for the last six months for my two year-old cat. I only feed her a quarter cup in the morning and a quarter cup at night. My cat has stopped gaining weight but has certainly not lost any even though I am giving less than the recommended amount. I'm sure the fact that she's an indoor cat doen't help out. The fact that my cat is no longer gaining weight is a good thing, and believe the formula works. I also had another cat that recently passed, but this was the only dry food she would even go near, so I think it is good for finicky cats too. I would recommend this to others who have cats with weight control issues.
IAMS Weight Control Cat Food is recommended by Daystime
Your cat might be depressed from the loss of your other cat. You might try some interactive toys - sticks with feathers on the end, or some catnip toys to get him moving.
I recently adopted 2 cats, to add to my 6 pack (I Know!) and 1 is waaay big. I already used Science Diet Light formula dry food for everyone, for their better health, but this 1 guy isn't showing any signs of loss . . . He actually looks bigger now that he's settled into the family!