if you can't feed hay in the field, then you want to increase the SB and chaff and lose a fair bit of the mix. there's no point giving a huge bucket feed as a substitute for hay - it doesn't work.
the stomach of a 16hh horse is the size of a rugby ball, so a 13.2's is smaller.
if you feed more in one go than the horse's stomach can physically hold, then you are just wasting it and risking colic. the feed doesn't get broken down by the enzymes in the stomach as it gets pushed through by the new food coming in, so it goes on down the gut undigested and can ferment or go off. at best, you're feeding the muck heap (as my exam instructor used to say) and at worst looking at gassy colic. horses have small stomachs and long hindguts as they are designed to be trickle feeders and can't cope with few large meals.
so it's not terribly useful to stuff loads of food down in a short time.
can he come in during the day or night and get plenty of hay over several hours then?
as regards feeding - chaff and SB are both high in fibre. the horse uses fibre to keep warm - heat is produced as they digest it. i like to feed high fibre and low concentrate if the horse is not in very hard work (ie eventing) so i don't feed much mix or cubes. so if he were mine, on what you've said in this thread, i'd see if he can come in during the day and have a few slices of hay, and feed (for example) 1 scoop SB, 1 1/2 chaff and 1/3 mix.