The most popular fiber among folks here seems to be psyllium husk, either Metamucil, Konsyl, or in bulk. It’s worked well for me for 19 years, but some folks don’t care for it. I haven’t heard of it before, but there are two possible mechanisms that I can think of that might conceivably be related to his muscle cramps: 1) fiber supplements will change the water balance in the stool, so someone whose hydration is marginal or whose electrolyte balance is marginal could be pushed passed a tipping point; 2) Some of these will bind with some medications, reducing their effective dose - so consider spacing medications a couple of hours away from fiber intake.