How do you know he's prone to founder if he's never foundered? What does you vet say about his grass intake? I would talk to my vet, for sure, before I'd count on advice from a forum. We don't know you or your horse, and your vet does.
Anyway, the best food for horses is hay, but if you feel he's prone to founder you should monitor the type and amount of hay he eats. Twenty-four/seven pasture sounds like too much for a founder-prone horse unless it is sparse, low-quality grass. Rich, sugary spring grass is a definite no-no. I would suggest taking him off 24/7 pasture and putting him in a dry lot, then feeding him the proper amount of hay in at least 3 meals a day. It is hard to evaluate the amount of grass he's eating if he's in a pasture.
HORSE JOURNAL says to determine the proper amount of hay you give to a horse by taking 2% of his weight as a starting point for the TOTAL amount of hay fed per day. In other words, if he weighs #1000, taking 2% of that, #20, and divide that by the number of meals per day. I feed my mares more at dinner time--b/c it's longer from dinner to breakfast than breakfast to lunch--so at #20 I'd feed #5 at breakfast, #5 at lunch and #10 at dinner. Depending upon his work level, add or subtract from the day allowance: maybe go down to #18 total per day if he's getting chubby or #21 if he needs more weight.
Runs this past your vet before you change your horse's feed. He/she can suggest the type of hay to feed, based on your horse's metabolism. Good luck.