I have read many of these responses, and while the doc and many other moms say this can be normal, I had an experience much like "Barri" relates.
My oldest son, from the time he was a year old and transitioned from regular baby foods to finger foods and toddler foods, became VERY VERY picky. He would no longer eat most fruits/veggies - he spit the chunks out of the Stage 3 foods and Toddler stuff. Only solids he would eat were cheese, crackers, meats, and occasionally bananas and grapes. He also LOVED his milk - when we stopped formula, we gave him whole milk with Chocolate Ovaltine for extra nutrients, and also continued to give him the PolyViSol vitamin drops the pediatrician had recommended. What I didn't realize as a new mom, was that all that dairy and the iron in his vitamins, and the lack of fiber from veggies and fruits was seriously constipating him. He started this well before we started potty training him. He would not poop for 3-4 days and then start crying and squatting and we tried to "walk it out" around the house. I tried suppositories and they usually came back out without having time to work. We dropped the iron-foritified vitamins and used a children's laxative (I think it was the juvenile version of Senokot - I can't remember from 1997!). I mixed that in with his chocolate ovaltine, since he wouldn't give up milk and the laxative was supposed to be "chocolate flavored"! Ew!
Anyway, this helped somewhat, but by the time we did start potty-training, he would resist again, simply b/c he had always been standing up holding my hands as we walked the house to get things out. He could NOT sit to do the business! So the cycle got worse again. The scariest moment was when he was almost 4 and had finally potty-trained. He felt like he had to poopy again, about an hour after doing so. He went to the potty and pushed for a while, and then started calling for me, saying it was stuck again. (We'd had to use Qtips in the past to help pull it out! They tell you not to insert any objects into their bottoms, but as soon as I would see something, if his pain was bad enough, I had to help it come out! Horrible!) So I went to check and could see SOMETHING hanging out his hiney, but it wasn't poop-colored - it looked pink! I pulled him forward and realized it was skin - he had pooped his insides out! I totally freaked! I called the emergency number for his doc and they said it probably doesn't hurt him badly and that I just had to use my finger to gently push it back up inside. They called it a prolapsed rectum, and said it rarely recurs. I did it, and it didn't seem to bother him. They said his body had probably pushed so hard earlier that the nerves were still sending the signal that something was in there. Blessedly, it never happened again!
My son is now 12 and still only poops every 2-4 days usually. And is still picky about fruits and veggies - but we have found a few he will eat. I don't use laxatives anymore unless we have to, but it is rare now.
So study your son's diet, sneak things in - that suggestion about the Benefiber mixed in everything is a good one! I used to mix baby food pureed prunes into stuff - it doesn't mix well with much though! I had a recipe once for Devil's Food Cake cupcakes that you mixed pureed prunes into and you couldn't taste it. It worked - just made the cake very thick and heavy - not cakelike and of course, my son didn't like that texture, so it didn't work too well for him after all. But the taste was fine! Oh and if I cut up bananas onto All Bran cereal and let the cereal get really soggy with milk, he actually ate that for a while! Yuck! :)
Find whatever method will work gently and gradually - alot of times we ended up with diarrhea from too much laxative stuff and that made him hate pooping even more!
Probably more info than you wanted, but if even one of these experiences/suggestions helps, I am glad. Sometimes what other people think is normal is not even close to what you are going through and just waiting it out will only make it worse. Use your judgement to follow whatever advice makes sense and seems to work. Breaking the cycle soon will be better in the long run! We suffered for years, and occasionally still do, so do what you have to for your child!
Blessings,
A.
PS - As a side note, my second son didn't like milk, loved juice and even though I watered it down alot, he ate ALL THE TIME, and just about anything! As a result, around age 15 months for about a year or so, he had what they call Todder Diarrhea! 7 or 8 very messy diapers a day!!! Talk about your extremes!