When do I know when to increase the amount of formula that my baby drinks? He is 6-months old and only 13 pounds.

7-8 oz average now. Most babies would be drinking about 7-8oz bottle at this age and 24-30oz a day along with some baby food would be average for a 6 month old. Your child is on the small side --feed as much as he can consume and increase slowly. Also, he shoukd be followed by his doc regularly. Good luck.