How to help my 2 and a half year old with constipation?

Prune juice & fiber. For little kids, start by increasing the fiber in their diet - bran, oatmeal, & other whole grains. Prune juice can also be very helpful. The key is to do this every day, preventing the constipation in the first place. It's much easier to keep them from getting constipated than to treat them for it afterward.
May be holding stool. One of the most comman reasons for constipation in a 2-year old is either anxiety or fear of pain from the bowel movement. The stool may be too large or hard in consistency. He might have a painful anal fissure and the fear of pain when he goes might cause him to hold it. A fissure is a cut at the anal opening as a result of a large or hard stool.