Just wait until he counters. He'll either agree to your current deal, he'll reject your current deal, but say he's willing to sign more, he'll reject your current deal saying he's going to college (but will still be availalbe to offer a contract to), or he'll reject your current deal and you'll have no more options to offer him any contract (I think this is more common when he's represented by his mother vs an agent)
Under scenerio:
1: he signs
2: he'll sign for the raised asking price the cycle after you offer him the raised bonus
4: He's not signing and there's nothing you can do about it.
3: He's not signing, but the good news is you can go thru the process again and again until he does sign. I had the #5 pick in a recent draft where I was able to land the top pitcher in the draft. It was a team I took over mid-season last year, and I'm hoping this is the last time we have a top half of the draft pick any time soon so it was very much a needed signing in a great draft where I doubt I'd get an equally skilled player next year w/ the compensation pick (would be the 6th pick). Anyhoo, after rejected my first deal after about 7 or 8 cycles, he rejected my 2nd offer after about 6 cycles, then took 21 cycles or so to reject my 3rd offer, then finally on my 4th attempt, after about 6 cycles he said he would sign for slightly more bonus money (about 5.5 mil vs the 3.5 mil he was originally asking for).
I think as long as they're willing to negotiate, you can keep on offering and they'll eventually agree to sign for some amount of money.