Your best bet is adding corn oil---it adds energy (read calories)
without making them nuts. Start out with just a glug or two in his
grain to get him used to it, as some horses don't like the greasy feel.
Others love it. Increase the amount until you're adding maybe a cup a
day---you may need to take maybe ten days or two weeks to get to that
point, use your own judgement on how much he'll accept. Judge for
yourself on whether or not he's at the right weight. If you're feeding
him four quarts, that's probably around five or six pounds more or less,
so you can easily replace 10% of that amount by weight with oil, which
is .5 pounds or two cups. Two cups of oil per day will add around 2
Mcal of energy per day, equivalent to another couple pounds of grain.