Welcome to the world of chronic constipation!!
My son has gone as long as 9 days! He holds it so well that he has stretched his entire lower third of his digestive track out of shape. We have been dealing with this for two years -- yes, you read that right, two years.
First, guess how long it has been since his last BM, and go talk to your pediatrician. If they are anything like ours they'll start you off with Miralax or something like that. They'll also tell you how often to give it, and a range to work within base on his size and diet. They'll also suggest changing some foods. Do it, keep a log and note when he goes -- note what he is eat too if you can.
Second, recognize that this is your son's issue and not yours. You have NO control. You have to let go. More kids are abused because of chronic constipation than you would imagine. It is really hard to watch your child doubled over in pain, or poop blood simply because they have held it sooo long -- and NO one will really help you. They won't until it is going on for months, and even then they are slow to help. They= medical people.
Eventually, if you are like us, when it goes on for too long and the regular pediatrician is useless, you will call a pediatric gastreenterologists and go see him -- it can take months to get an appointment. They give you hand outs and a different medicine to try. They if you are like us, you'll try several different ones, finally find one that works, and it will get recalled, and mess up your whole system.
The goal is to get the poop so soft and watery that the child goes once a day for several weeks. However, figuring out the right amount of meds is trial and error -- so you will most likely get explosive poop on occasions. This is really gross.
We now have a system and our son goes ever 2 to 3 days and we can live with this. I have no idea if he will ever get off the laxatives, and I know this is bad for him. However, it is way better than the pain, and blood that we were getting.
That that you are totally depressed, :-) Couple of tips --
After you eat there is a natural reflex to poop. Have him sit after he eats, his biggest meal of the day. We would read to our son on the potty for 20 minutes after dinner. It helped sometimes. Also our son tried to avoid pooping to the point that he ate very little, so I got him a large M&M McFlurry and had him sit on the potty to eat it. He was in heaven with the ice cream -- he forgot to stop the poop.
Second, push fluids -- they help keep the poop soft.
Third, there is a kid book on this issue that might help -- It Hurts When I Poop -- here is the amazon url:
http://www.amazon.com/Hurts-When-Poop-Children-Scared/dp/...
I hope this helps. This takes awhile to fix and you need to hang in there. Good Luck!