8 wk old baby...how do you treat constipation?

My son is about 8 weeks old and is now eating rice cereal with his feedings. He's been eating the cereal for about a week and has had his regular bowel movements. Today he's been straining to have a bowel movement and every time he tries to have one he just screams and cries until the urge can p. Are there some recommendations on how I can help him out?