Thank you for your query.
As a mother you need to monitor your son's weight. If the weight is increasing as per the standard graph then you need not worry. Secondly, at this age you should not stress
on milk. The child should have every thing to eat which an adult has. This is going to provide all the required nutrients and micronutrients for his all round growth.
I hope I have responded to your query to your satisfaction.
Wishing your son the best of health.