A: Mabel, it’s good that you’re concerned about your son’s nutrition and growth. For your son’s age and height, the average weight would be 29 pounds. At your son’s weight of 25 pounds, his Body Mass Index is less than the 5th percentile (out of 100), which indicates he is very underweight. Although some babies are naturally chubby and other babies are naturally thin—which can be healthy and okay—the fact that your son is very underweight could be a sign of illness.

It’s very important that you have your doctor check your son. Depending upon where you live and the quality of your local water and sewerage systems, your son could have picked up a parasite infection (sometimes described as “worms”) which may interfere with his appetite and growth. The doctor could check a stool sample for parasites. If he has parasites, then a medication could help him recover. He may also have anemia, or low red blood count, possibly from a deficiency of vitamins or iron in his diet. The doctor could do a blood test for anemia. If he has anemia, then a prescription of vitamins and iron, and more iron-rich foods could help him recover.

Don’t give your son any medicines without first having the doctor check your son and determine the cause of his being underweight.