Your demand letter to him has very little value in court, unless he acknowledges the accuracy or truth of what it says in some way; otherwise, it is just *your* opinion as the agreement and does not prove that's what he agreed to. Writings, including emails or texts, *from him*, and proof of payments made to and accepted/received by him, can be very useful evidence, but your letter to him proves nothing except what you are demanding and believe you are owed.