Just giv him teh alfalfa why risk the dangers of feeding Corn with it's KNOWN issues and the studies showing how BAD it is??
Corn is 70% or more sugars and starches ... he very well could have a mild colic going on that will only get worse
I say you ahve one smart little horse that doesn't like the cany you are giving him
I have been called the NSC Nazi more then once ... I hate traditional feed methods of loading our horses up on grains and junk food :)