First if your baby is on complete breast feed then there is no chance for constipation. Breast milk contains about 80percent of water. So wait for a week since the baby pooped last. It is normal for babies on breast feed not to poop upto 7 days.
If your baby is on formula milk, gently give him a massage frequently on his tummy using some massage oil or coconut oil. Search 'I love you massage for babies' on google and follow that. Then do cycling excersise. Be gentle while doing.