You are supposed to feed 3% of the body weight. 800lbs x .03 = 24lbs a day, =12 lbs a feeding. If you want to get the most utilization from the feed, feed three times a day at a rate of 8lbs a feeding. Feed every 6 hours ex. 6am, 12pm, 6pm. That should help with the weight gain, careful with the hay, if you don't feed enough he can bloat, but if you feed too much he won't eat his grain, so just watch his feedpan and belly and adjust accordingly.
You don't want to use fitters edge, you want to use the other one, I think it's just called finisher or something like that, same company, same bag just a different name
Our fair is the first week of august also, the calves weigh the same as yours does right now and they finish the same. Something to bear in mind. Stop panicing. You are going to make him sick and you will lose all the gain that you already have. You need to back him off and put him on a different feed, finishers edge, not fitters. Maybe give him a little baking soda to reduce the risk of bloat from all the feed he is getting. When was he last wormed?