What foods do you give him ,have you been spoon feeding him, at this age they like to have finger foods and be left to eat themselves, let him have plenty of time children do not like being rushed .spaghetti, chicken nuggets, fish sticks, yoghort, crackers , string cheeses, peas, beans , jello, grapes in small cut up bits put them in front of him , let him eat ..
if he keeps his weight fine, I wouldn't worry too much about it. Keep trying to get him to eat though, as he still needs that nutrients. If he starts losing a lot of weight, then you should take him in to be checked.