I know there are tons of questions about this, but i need help with senior horse

I have a gelding who is about 36 years old, i've had him for 13 years and I want to keep him as healthy as possible, every winter for the last 2 or so winters he's lost a lot of weight so this year i'm going to do everything I can to keep him fat, he is missing all but 4 molars on the bottom and all his top are gone(he has 2 bottom on each side but they are not next to each other)
He only gets grain once a day because with my son and work schedule I can only get out there around noon and then 5 and that would be too close together to feed grain(but I make sure he has enough hay to eat pretty much all the time) Right now he gets 2.5 3 quart scoops of Purina senior soaked to mush,with 1 scoop of alfalfa pellets also soaked. I went to tractor supply company and got him rolled oats, rice bran and beet pulp, I did buy a scale to weigh the stuff(i just keep forgetting to bring it back to the barn with me) My question is about how much of all of this should I be feeding? He is a 14.2HH arab/quarter who weighs about 800lbs(although I haven't taped him in a few months so that could be off) I also blanket him, he has a heavy weight blanket an attached neck cover and for really cold days he has a fleece liner on under it(lately it's been in the teens here with windchill below 0)
Thanks for any help you guys can give!

Total per day needs to be about 30lbs ...

I would change to either Nutrena Senior or Triple Crown Senior or Kent Senior ... basically any senior BUT Purina;)

Feed the Min amount of the senoir normally 6 to 8lbs

I wold start with 1/2 rice bran and build UP TO 2lbs

Then I would do abou t6lbs of beet pulp and 14lbs of alfalfa cubes well soaked :)

Actually I would go lazy man's way and feed max amount of Senior then soak about 10 lbs of alfalfa cubes

Thanks for the advice, I will look into switching him to a different feed, we get the feed directly from Phillips feed distributor because my mom owns a boarding kennel and we sell pet supplies, so we only pay 13 something for the Purina senior. I just texted her to ask her if we can get any of the feeds you listed above. If she doesn't get back to me I will ask her for a list when I go feed tomorrow

I've had amazing results with Nutrena Equine Senior.