Upload some proof to the Report a Scammer section and get him banned! If he decides to come back to Sythe, he'll have to repay you in order to be unbanned. Here's a link to the report-a-scammer section, good luck!
Are you speaking of a Runescape account? If so, you are still able to recover the account regardless of whether or not you gave him the recovery information. The original owner will always be able to recover the account.