Your son's growth is adequate. Many children are poor eaters. They may eat some foods better, but even then have small appetites. As long as you force feed him, he will never learn to enjoy food. As long as he is gaining at least 1 kg weight every year and is active, do not force feed. Instead encourage and present a variety of foods and try to get him to eat with you at the dining table. Appetite builds up slowly and develops by about 8-9 years of age- so do not worry. From your description, he sleeps enough.