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What you could claim is to have the contract nulled due to incapacity. A party is deemed not to have the capacity to enter into a contract if they do not understand the meaning and effect of the words they are agreeing to. The fact that other loan offices would not loan to him demonstrate that the loan office that did loan should have known of his incapacity to enter into the contract. The loan office took advantage of your son's clear diminished mental state. Usually being on drugs does not null the contract, but if the loan office knew of your son's condition, then you could potentially get your son out of the agreement.
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