'll try to sum up: My brother -- born very prematurely then got very sick -- has tourettes syndrome, is bi-polar, has learning disabilities and some other issues that affect his ability to comprehend the world around him (i suspect some autism). He worked for years doing odd jobs and saved enough to by a small convenience store, drove it into the ground because he didn't understand how to run it, and is now being sued, among other things, because he signed a contract he was unable to keep. The guy he made the contract with a few months ago is clearly trying to take advantage my brother's inability to understand what he was signing or defend himself now. i'm wondering if someone who is mentally incompetent can even enter into a legal contract if they don't understand what the contract says.
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.