My 11 y.o brother says it hurts when he "passes" because his stool is over-sizes. too big. huge. how can we break it up?

Sounds like: constipation. Recommend increased water/ hydration, ^ exercise and more fiber/ bran in his diet.

More daily fiber: like Raisin Bran for breakfast every morning.Fiber increases the water content of the stool and makes it more soft and passable.Also more daily water/fluids.

Stool retention: This is often a chronic problem that has been going on for years. The kid hates to go so his poop dries more and bulks up. Kids this age often have a distended colon that empties poorly. I would consult with a developmental pediatrician or pedi-gastroenterologist. A bowel rehab program that may take a year is likely. Short term solutions like an enema or fiber seldom work in the long term.