Fair question, with an unfair answer. The rules for the "Democratic National Committee" (or, undemocratic as I like to crack) is that his unspent funds are to be donated to the nominee. Which at this very point, is still undetermined. Considering the Hillary wikileak, the 20+ lawsuits and the fact that she's losing to Trump in all polls, it's not completely impossible the super-delegates will place their votes on Bernie, giving him the nomination at this contested convention. We will know very soon what happens next.