I feed all at night. He gets 4 cups of alfalfa pellets, 6 cups of oats, 2 cups of bran mash, and 6 cups of beet pulp pellets which are soaked so it is more than that.
Yup, sounds a lot to me too, esp for only one feed. What sort of condition is he in? How much hay/grazing does he get? Has he had his teeth done recently? Is he stabled? Lives out with mates...?
It's important to understand the basics of a horse's digestion and what they've evolved for. That is, 'trickle feeding' of high fibre, low 'cal' roughage. Their small stomachs are not built to cope with big & or infrequent meals and they don't cope well with starchy, rich feeds. If it may be necessary to feed ingredients such as that, it is all the more important to feed it little & often, or else there will be a flood of starch to the hind gut, which can cause real problems, including ulcers, colic, weightloss, laminitis, etc.
So I'd personally cut out the oats, for a start, especially if you can only feed once a day. Interested to know why you're feeding a bran mash & what kind of bran? If it's wheat & there's no specific nutritional reason he needs all that extra phosphorus(he'll be getting a lot from the oats too), I'd cut that out & feed an extra calcium supp for a bit to help correct the balance. Do you feed any nutritional supps?
For more info on feeding horses, safergrass.org, feedXL.com & hoofrehab.com are among some good places to start. I think it's a good idea to educate yourself & get specific advice from a nutritionist or such, rather than a feed merchant or just 'someone's' opinion... after all, there are acres of different opinions abounding & not all of them are suitable, regardless of the 'experience' of the person.