In CA, the statute of limitations on an oral contract is 2 years from the date of breach. if you've been paying the loan back, unless your oral contract was that you were supposed to pay more than $100 back per month or pay more frequently than monthly, you haven't breached the contract.
If you have beached, or you do breach in the future and he sues you, you'll most likely be liable for repayment of the balance owed. If there was no agreement for you to pay interest or late fees or legal fees, you won't owe those, and if there was, you will.
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To add to my colleague's answer: The 2 years does not begin to run until after the last payment you make. Every time you pay him that $100 it gives him two years from the date of that payment to sue. This is a small claims lawsuit.
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