Capacity to contract - the basic ability to understand what's being contract for - is a prerequisite to any contract. So your brother;s contract may not be enforceable.
See a lawyer to disclose all the facts and get this specific contract reviewed and get some individualized advice.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.