Miracle,
honey water and barley water are typical remedies for constipation. but yr bb still very young leh, better dun give. and like wat singapoh said, honey is no no for under one.
did u try changing FM brands? some FM brands are more heaty and more tendency for bb to get constipation. Can try mamil or friso, which are supposed to be better for constipated babies.
if u bring to pd they will also advise to change FM first, last resort is soy milk.