Feeding plans

I have a 726 lbs steer that is on corn and protein. How much should I be feeding him daily?

Completely depends on when your show is and how big you need him to be. As a rule of thumb, most cattle will eat 2.5 to 3% of their body weight when on full feed. So your steer could be eating between 18 and 22 pounds of feed a day. But like I said depends on the show date and target weight.

My show is the end of july

I would keep him eating as much feed as he will eat. Also , be sure to provide him access to hay.

Are you just feeding corn & protein? 

yes for right now

He needs a complete feed; a finisher of some kind.  You need to talk to your adviser of feed dealer close by and find a good complete feed.  Not sure how he is going to look on corn alone.