It is really hard to go from a big horse to a mini as we all want to overfeed them. IT just doesn't look like enough food!
If I were feeding 31 inch overweight horse, and this is really not enough info to go on, I would probably be feeding 1 pound of hay twice a day, I feed alfalfa, and a cup of a well balanced pelleted grain, I feed a 12% Strategy.
Again, just giving us a height is hard to give advice. Is he a quiet horse, does he pace, is he nervous, etc.
I also would try to get some esercise in even if it meant early morning or evening romps.
Remember and I am sure you know this from horse experience, it did not go on fast, it has to come off slow as well.
Play with the amount of feed and keep cutting back until he starts to lose. When he gets close to where you want him start upping the feed slightly until you have him maintaining a healthy weight. Get it down now before he gets hairy this winter as it is hard to tell what they are really like under a fluffy winter coat.