If it is from constipation, I know that organic (or the Gerber baby brand) apple juice will help loosen baby stool. We used a 1/2 and 1/2 mixture on our DD when she hadn't gone in a week (she was only 6 weeks old then, still with the loose baby stool), and the next morning she just gushed (it was just like water, soaked the diaper through). And prune juice would probably work too (although your son might not like the flavor).
I've heard of using flax oil, but I'm not sure how much you'd need to use.
I've heard of using flax oil, but I'm not sure how much you'd need to use.