How is his height and weight gain? if they are fine, then dont worry about the food intake but make sure when he eats its healthy and home cooked meals. Avoid giving snacks or treats in between meal times. For the behaviour part, make sure when you come back from work, spend some time with him and play (u can play with him what he wants like making somethings with block or throwing a ball to each other, or sing some rhymes with actions for him). you can also involve him in small house hold chores like setting the table or watering plants. Do not forget to appreciate him for the things he has done.

If his weight, height and development are ok, there is nothing to worry about food intake. Children grow slowly at this age and eat less food. Try to spend more time with him. He should get better when school starts. Consult a child psychologist.