A.P.
I'd request your pedi. do a urine and fecal assessment.
FYI - it might not be the case for your son, but my 4 year old has tried those chewables before and said they made his tummy hurt - which could be the reason for wanting to lay on the couch - cramping.
The reason I suggest labs would be because poop is a great indicator that something is amiss in our body! It could be blood sugar, it could be a misfuntioning organ, it could a number of things.
Hemmoroids can be seen on the outside of the anus. If you don't see anything when you help clean him, then its not a hemmoroid, but could be a polyup or a fissure (which are inside on the walls of the rectum). Is blood evident in the stool? That's usually a sign. Those, depending on the size and severity, can be treated when they flare or removed by a doctor. Again, ask the doctor what he thinks and go from there.
Are there fevers along with these bouts of consitipation? Is there gas with the constipation? Are the stools hard and large or just infrequent?
A great piece of data you could provide to yourself and your doctor would be a food/poop journal. Keep a journal through an entire regular/constipated cycle. You might be able to find there are particular foods/combos. that are causing a reaction.
Probiotics are great! You can get them at health food stores and pharmacies. They help to keep the good bacteria in your stomach and will help keep BMs regular. These can also be sprinkled into yogurt or applesauce.
I agree with another poster that people over-use milk when our kids become toddlers! 2 cups (16 oz) is all a 2 year old needs. A toddler should not be walking around with a sippy of cold milk, but a sippy of water. Milk tricks our tummies into feeling full so we don't eat as well. Also, it does constipate. Whole milk is bad for your cholestrol. Bananas are constipating too - so if he eats them daily that could be a culprit.
In my experience, I get more from my doctor when I show up with some data and a list of questions and requests than if I just walk in and say he's not pooping right. The more info. you can gather ahead of time is very helpful to the doctor (and may even give you the answer and save you a copay!)