For what it's worth, Justin has coliced 8 times in 10 years...most were gassy/grassy colics when the grass would get too green too quick, but he has had 2 impaction colics. After the last one, I said "no more!" and now he lives on soup. He has always been known to not drink enough, so the extra water really helps. Your guy probably won't like the change in texture at first, but if he gets hungry, he'll sure enough eat it.
For every 3 pounds of grain Justin gets, he also gets at least 2 gallons of water on top...his poops are consistent and well formed now...and I can be sure he's getting the adequate water intake he needs (3 feedings of grain = ~6 gallons of water + 2 feedings of alfalfa soup = ~6 gallons of water)
Hope he is still doing well!