I am so sorry your little guy has to suffer with this curse. I'm afraid I don't have any suggestions to make to ease his pain, and I really can't give you any answers to your questions. Personally my problems started with a botched appendectomy at 3-1/2, but I never experienced any real intolerable pain until about 4 years ago. In discussing my current situation with my mom, she reminded me that I have always had a "slow bowel", i.e., I didn't "go" every day, but about every second or third day.
We have several members on the Board from Australia, and possibly they could give you more pertinent information regarding your area, being closer to you.
Sorry I'm not much help on this, but please know that your son is in my prayers.
Nancy in NB