Typically, the big belly with ribs showing is an indication of too little protein and or too high fiber forages. Adding 1-2 C of oil a day will help add a layer of fat but it will not add meat to his bones. I'm not familiar with either feed but what he needs is a good quality protein source. You could add soybeans, alfalfa or a commercial feed (30% protein). The amount of feed you are feeding is also very little (about 1K) so unless it is a concentrated source of nutrients, you probably aren't getting much value out of it. If the belly is a result of high fiber hay adding a pre/probiotic daily will help break down the long stem fibers. He also needs work to build up the muscles on his topline and prevent the sagging belly.