Please Help!! Thoroughbred is skinny! Dont know what eles to do!?

Hi! I am going thru vet tech school, so my thoroughbred has had his blood drawn, teeth floated, beaned, and fecal checks routinely. He is 12 yrs old. And a complete pasture ornament! Not doing anything! He shares nine acres with one other gelding. Fed twice a day, xfactor 12 protein 8 percent fat. corn oil poured over it, weight gainer supplement on food twice a day. alfalfa at night. free choice hay.. as well as full turnout on the acreage. he is fed two coffe cans twice a day. four cans a day. which in pounds is to the max of what he can be fed at a time. my paint is fed 1/4 can twice a day and is fat. I am at my wits end really! i am tired of people asking me if i feed my horse. please help! ps. he is a very slow eater.. and really picky. and yes, he is seperated at feeding time.