OK before suppositories (I know docs LOVE to use these but...)
You need to figure out why he's in pain. If his tummy is soft, he might be in pain due to a stomach upset from something you ate that passed through the breast milk. He may just be a little gassy, which the mylicon drops and tepid water in a bottle will help. He may also be finishing his 8 week growth spurt, and the lack of poop is just a side effect. Babies get growing pains to!
If his tummy is hard, that means severe gas and/or bowel block up. Warm baths belly massages working his legs. When adults as backed up, doctors tell us to move more. Some babies can also receive 1oz prune juice 3oz water (if your doctor OKs this) via a bottle.
Use OTC suppositories as a last resort. Not only is it better for their bowels, but suppositories can have unintended consequences on babies (such as projectile poop). I've worn baby poop before because of suppositories - it's not pleasant. =)