Don't go for a mediator: mediation is voluntary, not mandatory, and can't compel an outcome. You could spend a great deal of time and money on mediation and get nothing. At this point, you need to sue for breach of contract and/or fraud--taking your money without doing what he promised he could or would. Retain an attorney who handles business or consumer litigation (that is, lawsuits arising out of violating contracts or agreements, or from false commercial advertising; not a lawyer who goes to court but only on family law or personal injury lawsuits or criminal cases).