Nowadays the milk powder are very "hot". If you found BB便秘, not only no "patpat" come out. BB himself feel very "hot" inside, tummy/intestive not good as well.
All kinds of fruits will help..
How old is your BB.
Try to have a very old apple, which inside the is very soft and tender, using a teaspoon to "scratch" to BB, eat at little by little.
Some drops of orange juice can do as well.
Using old style "congee" water 粥水, to make milk can help.
Some more natural way.
You need to let BB had a habit to "Mmm Mmm" everyday.