Hi and welcome to HCM!
Thanks for posting your query here.
The age of your son is not mentioned. Anyhow I will answer to the question. Indigestion
and passing partially digested food particles shows a problem with the digestive enzymes. A recent changes in the diet like adding cereals, legumes or change of milk or milk powder could be the reason. If the baby is active that is a good sign. Please check the food giving to the baby. If you are introducing a new food to the baby give one food at a time. If he is ok with that we can continue with the food. Any indigestion symptoms appears that specific food can be avoided. Please dont mix 2 to 3 new foods at a time.
A digestive enzyme syrup like Unienzyme- 2 teaspoonful 3 times a day can be used to relieve the symptoms. If still the problem persists please consult paediatrician.
Hope this is helpful to you.