He has always been on quite a volume of feed. He eats it all. He is just really picky. Doesn't like Standlee brand, was just tolerating it and now he is done with it.
Has he had his teeth done? Have you done a diet analysis/bloods for mineral levels, to see if iron overdose or such is a problem? IME when horses 'become fussy' there is usually a physical reason for it.
As for overall volume of feed, horses have small stomachs that aren't built to be empty or for large, infrequent meals. Their digestive system is built for processing tiny amounts near constantly of roughage. Food only stays in the stomach for a relatively short time, but it also empties when it's about 2/3 full, regardless of time or digestion. So your horse may well eat it all, but being fed a large meal will likely mean that it travels through the stomach, into the hind gut with very little digestion.
Large amounts of feed hitting the hind gut suddenly can be problematic, particularly when it's starchy - ulcers, colic & hind gut acidosis are 3 common results, along with 'secondary' type problems this causes, such as failure to thrive and laminitis. Feeding little & often can greatly reduce the risk of these problems. If you can't feed little & often, small, low starch/sugar/grain meals are especially important.
according to the bag of senior grain he should be getting more than I am feeding.
Of course they recommend a lot more than any horse generally needs - they want to sell more product! If you're going to use a nutritional advisory service, I'd also avoid those working for or affiliated with feed co's, for a more objective, impartial view. I personally think feedxl.com is a fantastic service & you also have access to a qualified nutritionist for any particular questions you may have. As an aside, it has actually saved me money, as I can *know* the true analysis of feeds & supps & easily work out what & how much is required for a well balanced diet for any particular horse, rather than overfeeding stuff that isn't all the packaging/co says it is.
I am going to be switching him to Triple Crown senior which is a lower starch feed. I just have to drive 45 min to go get it
That seems to be a reasonable quality feed, which would also be best fed with extra roughage & in smaller meals. How about asking if your local store can get it in for you, or asking TC if they deliver in your area.