First of all, I hope you offered him exactly what he was asking for, as there is no benefit to offering more.
If that is the case, there are now three possibilities: 1. He eventually signs. 2. He comes back and asks for more money. or 3. He says he is not going to sign. (Going to or back to college, going to another sport, whatever.)
If #1 happens, congrats, he is yours.
If #3 happens, you will get the pick immediately after what would be the original #12 pick in next year's draft. (Exact number will vary based on higher teams in your situation.)
If #2 happens, you have the following choices: A. Offer him his new demand (and no more) and he will eventually sign. or B. Do nothing else with him and you will have the same pick as stated for #3.