Is it pulp or pellets??
I pulp I would soak in boiling water until it's cool enough to use- mine really like it hot, you know your horse, though if it is pellets you will have to soak until the pellets are completely broken down- mine don't like nasty hard things to chew, and it has been said it could cause choke it they were bolted down- they really are very hard.
Again you know your horse.
I would start him off with a cup or so soaked and put it in his feeder before you feed him so he can poke it around, make all his "I don't like this new muck" faces, and generally be crabby about it, then dump the rest of his feed in on top and I think you will find it will all go!!
I have never had any trouble with it but then, I have never had a shy feeder- well, one, last year , but that was a tiny foal!!
Just keep upping the amount until he is eating- I would suggest about five cups dry weight, soaked- how much do you think he weighs, roughly?? Rabbit is actually eating that much and he is old, but very small, so that is certainly not going to be too much for your chap.