I have never had a constipation problem -until now that I am on antibiotics!
I couldn't go in the morning, and it was painful. I took a couple large glasses of the V8 veg/fruit juice noni berry in the morning and by the afternoon I was doing a huge amount of softer poo. I was thinking.. for myself, If I bought a juicer it might help. I do know apple juice can give people very soft bm's and can even cause the runs!
Even tho your son doesn't have hard poop, he can still have a form of constipation. A huge amount of softer poops mean that the colon and intestine were quite full. Usually those come behind a harder dried stool that is blocking things up.
The other thing I would suggest is magnesium citrate. Maybe you can get it in a powder, and maybe put it in some juice. An adult dose is 200 Ml. I would take it at night because it helps you sleep and relax.
Magnesium deficiency will cause constipation, nervousness, anxiety, hyperactivity, ect. Magnesium will help with normalizing tension of the colon walls and draws water to the colon, relieving backup. It is not habit forming.
Leaning way forward while trying to go may help too.
I think if you add magnesium to his diet and some juice, you will see him going more often than every three days and feeling much better.