Talk to your pediatrician. He or she can examine him and either make suggestions or calm your worries.
Also, what is he eating? Formula or breastmilk? When my baby was taking breastmilk, he only had a bowel movement every 3-4 days. Once we switched over to formula, he has a bowel movement once a day and usually right around the same time.
What are the stools like? Are they tiny, hard, dry pebbles? If so, that could be a sign of constipation. Is it a thick pasty mass? If so, that's not normally constipation.
Please do not give your child anything to 'soften it up' until you speak with your pediatrician. In the beginning I bet your baby had a few bowel movements per day, but now that he is older it is normal to only have one.