Help Please! :)

Hey there all you beautiful mamas I've got a question and hoping maybe some of you all will be able to help, my son is 5 months old and his doctor gave me the ok to start cereal and solids. Also he's been a breastfed baby the whole 5 months except recently I've introduced formula due to my milk dropping. He does breastfeed most of the day but he does have 2 bottles with formula. But anyway my question is  every since starting cereal/solids he'll go a couple days w/o pooping. What can I do to help him get regular again? I've asked the nurses but they all tell me it's normal? Thank you all for reading my long story haha any advice would be greatly appreciated