I'd wholeheartedly recommend any one of the ration balancers, whatever you can reliably find in your area. Purina has Enrich 12 and 32( depending on what hay you feed), Nutrena has Empower Balance, Triple Crown 30% is another, plus a bunch more regional one's. You feed between 1 and 3 lbs daily, plus whatever fat supplement you need to add for more energy. IF needed. Plus the flax. And the BOSS. Which I think is an overkill, the Natural Glo provides quite a few calories, flax is high fat, too.
I mix everything in a handful soaked alfalfa pellets.
If you're not familiar with ration balancer I have a link to learn all about them. Just let me know