Probably not, I'm afraid. Most horse's diets are deficient/imbalanced in a range of nutrients, and if he's on hay only rather than fresh grass, hay also loses vitamins when dried. Different areas/soils/feed types have different lacks & imbalances too.
From looking at their product sheet, the Patriot stuff may well provide well balanced nutrition, but even if, that assumes you're feeding the vast quantities they recommend, which esp with the corn high in the ingredients list, I wouldn't personally advise even if he needed all that extra energy. As for amount of hay, even if he needs 'dieting', horses still need small amounts going through their system near constantly & the 'rule' is they should get at least 1.5-2%bwt daily in forage. Telling you that for 'Justin' because 4 flakes daily depends on the size of the flakes!
So I'd just stick to the hay or similar and include a suitable good quality nutritional supp or (grain free) 'ration balancer' or such. Feedxl.com is a great resource for balancing diets & working out the confusion of nutrition.