It could be due to formula milk if ur LO is not breastfeeding. Try to dilute the formula with more water or give him some mashed banana daily. It helps. :) If it’s solid food, what are u giving him? At 9 months old, u can start to feed him with some boiled water daily as it will also help to relief constipation.
hi mummy.. my boy is 7months and sometimes he got constipated too.. normally i will give banana / papaya puree.. and do massage and exercise to hell his bowel.. you can check this link https://wholesomebabyfood.momtastic.com/constip.htm