So you're looking at showing him at 15 months. If he's not very big framed, he may just be doing all he can do at this point. Most should have a little more in 'em though, so here's what I'd try if I were you;
I'd try to get the implant done this week, then - it should still help some. I'm still thinking you need to be feeding a good quality show calf finishing ration. Start mixing it half and half with what you're feeding now, then gradually work up to feeding entirely finisher in four or five days. He needs to be eating all he can eat at this point, so if he's cleaning up his feed, increase it to all he'll clean up in twenty minutes. The cooked corn and/or barley thing can help, but I haven't personally done it, but others here have, so someone else can help you better than I can with that.
Good luck, and keep us informed on how it's going.
David