Once you've started feeding the Enrich & flax (and free choice hay if possible, you'll see optimal results this way), give it few weeks to see if you notice any coverage starting over the ribs. If my assumptions are correct then this should be a sufficient amount of feed, but if he's a harder keeper than I suspected you could add a few cups of beet pulp or alfalfa pellets to the mix, you'll just have to play it by ear.
I also converted the weights I suggested into cups for you:
2 lbs (4 cups) of Enrich 32
1/4 lb (3/4 cup) of whole flax seed
I apologize for such long, ranty posts. My horses have been on the same feeds for so long I think I was deprived of playing with different ideas...