My 15 year old indoor cat has been losing weight over a 6-month period and the vet is offering suggestions - but so far nothing is working. Here is his history:

• he used to have 3 fixed meals when his brother was alive. Felt very stressed - but ate. Had to separate the two (the other was food fixated);
• when his brother died about a year and 1/2 ago - let cat graze. At first OK;
• now, he eats 1/2 of his food during the day and other 1/2 at night. When cat sitter comes in when we are on vacation - all is OK;
• when we are home - cat will eat very little at night (regardless of whether we have him in the bedroom or keep the door shut (with him outside);
• have tried putting another food bowl at night upstairs; also extra brushing (can't do treats because of food allergies).

Suggestions?