Zac, no one can tell your steer is finished without at least seeing a picture or better yet putting their hand on him. Just because he's 1200 lbs by no means indicates he's done.
I think Purina Power Fuel if he feels close. If you feel you need a little extra mix in some stabilized rice bran with the PF.
If your gonna use both, slowly bring him up on PF then slowly add in you rice.
IMO, its pretty darn hard to get one to fat. Especially feeding through the summer. G-Luck