That's what I was thinking... he doesn't 'need' the nutrients from the grain anymore.
With regard to the too high in nutrients comment, that was a generic type of possibility. As sweetfeeds are generally just junk food, they don't tend to have a lot in the way of nutrition in them anyway, so I don't think that's the prob here.
Grain is to help him keep weight since I work him?
OK, so he is a bit of a 'hard keeper' &/or you work him quite hard? It is possible that the grain is part of his weight problem, as the same probs that lead to ulcers & laminitis, can cause weightloss.
and the feed supplier is very educated when it comes to this stuff and he has given me the okay on it.
Well, you'd kinda expect that, if he's the one being paid to sell the stuff! Grain is not necessarily bad for horses, but there are problems associated with high starch feed that means I'd personally choose otherwise. If for eg. you can only feed once or twice daily, I'd really think about alternatives, because it's important to feed especially starchy feeds little & often to minimise the risks & maximise the benefits.
If he needs a high energy feed, ingredients such as beet pulp, soy hulls, rice bran are high energy but low starch/sugar. Katy Watts | Safergrass.org
has some good info on feeds & feeding. But considering what he has been fed & the probs, I'd also give him a probiotic and consider getting him scoped/treated for ulcers. I'd also be inclined to include a good nutritional supp or 'ration balancer' with his feed, so he's also getting good nutrition.
...& after seeing that pic, looks like he doesn't need any extras for weigh gain anyway, but could maybe afford to lose some.