Do you have a fat cat? Is he grumpy?

6 year old neutered male, stays indoors. He is very fat, 20 lbs. I have had him on a diet for almost a year and he hasn't lost any weight. I took him to the vet because he was acting strange and was very grumpy. I thought his teeth were bothering him. The vet told me he is grumpy because he is so fat and miserably. I got him a playmate to help him exercise, hoping this will help. My question is, do you have a fat cat? If so, is it grumpy?