i wouldn't feed haylage either; much better to feed hay thats been soaked for 12 hours to remove the soluable carbohydrates & remove a lot of calories, but you also need to feed a vitamin & mineral supplement to make sure he has all the nutrients he needs.

I have a native pony who has EMS so diet is very important; i feed Top Spec Antilam and oat straw chaff, that has no additives at all - only oat straw, so no molasses, oil or any other additives to add calories, plus restricted grazing. She has the same diet if she does get laminitis and is on box rest, but then has more of the chaff & the soaked hay is spread out through the day.

I have a native type suffering with EMS and who has a very severe dust allergy so have to feed haylage.  The reality is most hay has higher sugar levels than haylage as some of the sugar is used up during fermentation.  It is a trade off between how wet it is (makes it more acidic so not good for laminitics) and how dry (increased sugar).  Get your haylage tested and providing the sugar level is below 10% then it should be ok but be careful as it is more nutricious.

I also feed Simple Systems Lucie stalks which have no added sugar and give him as much as he can eat.  Although Laminitis Approved Happy Hoof, has molasses added to it, at least Hi Fi Lite it is molasses extract so check your ingredients labels.

Does he have to have haylage? Can he not have Hay. Haylage has way more sugar & calories than Hay. Id feed him Hay thats been soaked for a few hours. People say it should be 12 hours, but i personally find it sometimes goes a bit yucky after 12hrs if its not the best quality. Soaking it removes all the dust and lots of calories. Hope he gets better soon xx