Its most likely due to Reflux. You should not make him lie down immediately after feeding and instead try keeping the head end raised even during normal sleep. Second, try giving him cold
milk regularly and you can also give him Domperidone
drops regularly for 2 weeks. It will help in improving the tone of oesophageal sphincter.
As he grows up, try avoid giving him hot and spicy foods, or colas and other beverages. Fresh fruits and vegetables will be best for him.