Did he weigh 875 full or empty? do you have the ability to weigh him at home?
I would try and get an empty weight and a full weight on him first, see where he is at.
If it was full then you have a little more room to play. if it was empty I would feed him a good grass hay 6 pounds of grain and 3 pounds of beetpulp a day, plus a protein supplement like soy pellets and a fat supplement like "Fast fat" or something.
Weigh him once a week, on the same day at the same time every time and track his daily gain, if he is gaining to much cut back on the grain if he is gaining 2 pounds a day keep feeding the same. If he is not gaining, add more grain. Monitor him weekly.
The day before weigh in, first thing weigh him and see where he is at, if he is at or bellow 1000, feed him normal in the morning, bucket water him 3 gallons with electrolytes. That evening give him his hay and grain and 1 gallon of water with electrolytes. The morning before weigh in, weigh first thing in the morning and see where he is at, if he is 15 pounds or more below 1000 give him a gallon of water with electrolytes and hay.
If he is less than 15 pounds under give hay.
He should not be over if you have been managing his weight the whole time.
After weigh in give him between 2 to 3 gallons of water and some hay and some pro-biotics paste, an hour later another 2 to 3 gallons and a little grain (3 pounds) that night you can give him his normal ration he had been receiving on his diet.
You will need to slowly bring him back on full grain, it should take you about 2 to 3 weeks to get him eating a full ration. Take your time and monitor him as you increase his grain, if you go to fast you'll mess up his guts