DS had more constipation issues when he was about 1 1/2 years old when we switched from formula to whole milk. Early on (4 months old) with loose stools while still being constipated, or at least having a LOT of stool in his colon, so we needed to use milk of mag a couple of times. The issues with determining the amount of enzymes based on poop has always driven me batty -- especially when DS was a lot younger and not consistent in his eating. Sounds like he's doing well, and from what I understand, those who are on enzymes, still have some pancreatic function, so maybe as he's gotten older, his pancrease is working more efficiently.
And when we've had true constipation issues -- usually DS's tummy is a bit hard, he decreases eating, sometimes he'll tell us his tummy hurts and his stools are more like hard little bunny ones -- sorry tmi. Then we usually give him some milk of mag to clean him out. Most likely it that case it's again caused by meconium equivilant or stool backed up in the colon.
When we've changed enzymes, we've usually done it gradually over a period of time. Upped or lowered the amount for meals by half a capsule. He's been on 4 to 4 1/2 pancrease mt4s for meals for a long time. And snacks, usually 2 capsules unless he starts chowing down on something big or fattier.