I have no personal experience with a choking horse, but from what I do know, once they do choke, they can be prone to further episodes. Especially if it was a bad choke, it can leave the esophogus (spelling?) with scar tissue making it more difficult for them to swallow. It wouldn't hurt to soak his hay to soften it making it easier to chew and swallow, also if he has allergies it will keep him from inhaling the dust, so soaking can help both issues. Also, it would be a very good idea to soak his feed, if you aren't already doing so. You say he gets grain and beet pulp. The beet pulp is ok because it is fed soaked already, but you may have to rethink the grain, soaking may not make this any easier to chew and swallow. You may have to go with pellets or something that can be soaked into a mash like consistency like the beet pulp. Obviously if you were to change the feed you would have to make any feed changes gradually to reduce the risk of digestive upsets/colic. Another thing that may help, if you stay with the dry grain, is to put a large smooth round rock in his feed tub to slow down his intake, he may chew his feed more thoroughly then.
Just some thoughts, I hope they can help out some.