When you bring your younger son to the doctor, you need to have this kind of information. From when to when he was an easy eater. From when to when he became fussy eater. Did this coincide with roughly the time be became constipated? Of course, children goes through phases of growth and appetite changes, it could be normal. But writing things down and taking that pooh diary will help when you speak to the doctor in the future, should such a need arise.
Anyway, passing motion is two elements. Physical (the diet and the bowels) and Psychological (the mental stress). In some children, if they have had constipation frequently, they become afraid. So they try to hold it in for as long as possible, and that makes it harder and even more painful. And then the cycle continues.
In the case of my son, I tried to remove the Psychological element. I gave him Lactus every night to make sure that he can pass smoothly, without pain and distress every day. And then I keep to a routine. Every morning, and if fail, every evening after dinner. So once we have a routine in place (about one week), I stop the Lactus to see if he can still pass. I tried to give him fruits that are good for passing motion - dragon fruit, kiwi, papaya... And then, for the day that he doesn't pass, I would give Lactus again. And I took extra effort to make sure that he keeps to the routine EVERYDAY. Now he is 5 and I don't monitor him so closely. Once in a while, he skipped, and then he will suffer (crying, screaming, mommy hold hands)... and that will be a reminder that he must goto the toilet everyday.
I have not heard about biogaia, but probiotics is for good gut bacteria, which may or may not help with softening of stools or passing motion itself. And those fiber supplements has a small disclaimer that says "must drink lots of water"... otherwise, it actually makes constipation worse!