Saw Palmetto helps increase appetite but I have never tried it personally. Just be sure it is the berry and no other part.
A thiamine deficiency is often the cause of low appetite. Take a thiamine supplement along with B-complex, vitamin A, zinc, niacin, and biotin. These work well together and help in the assimilation of the thiamine.
And even try this. Find your favourite thing to smell. Before you eat, smell it for a minute or so. Appetite can be a thing of the mind and when you smell something you like it makes you happy. When your happy you like to eat. Thats why fast food resaurants make sure the air around the restaurant smells like the food - it makes people want to eat.