My son of about 3.5years old lossing weight though he was not sick and he eat moderately. what can i do for him.

See below: Get him checked by his pediatrician. There are many reasons why kids can loose weight; some minor and others significant and there is not enough space here to discuss the possibilities. Again, see his doctor.

Weight loss: Weight loss in a 3 and half year old child needs to be thoroughly evaluated. First and foremost, any acute/ chronic conditions need to be ruled out. Once all results/ data indicate towards normal healthy child, then look at his overall growth pattern. Is he aging height normally? If so, then routine diet with multi- vitamins should help. Consult your pediatrician.