As long as he is still sick and/or underweight, kitten food would be a very good idea. If you are worried about a change in diet upsetting him you can gradually switch over - 3/4 of what he's eating now mixed with 1/4 kitten food, then half and half, etc. until his diet is completely switched over. Chicken and rice chopped fine or pureed is also good for convalescing cats as a supplement to the kitten food.