My Barnzi (a rather odd lilac tipped persian) used to refuse to go outdoors, so set and supervised feed times were a breeze. I fed him on cod, tuna or prawns in the morning and purina in the evening, always maintained the same weight. When I moved, (he was about 8) he decided to start venturing out. He put on quite a bit of weight. I wasn't concerned, and I used to smile at him and a grey cat running around together and attempting to hunt all day.
When he started to go off his food, I found out the nice lady in the next close was feeding him too, Barnzi saw it as his second home. She was feeding him salmon and leftover turkey and chicken, and was even ordering scraps from the fish shop for him!
Are you sure he hasn't found day care elsewhere!?!