In PA the small claims court has a jurisdiction limit of $12,000 which means that is the most you can sue for. Since the amount that is still owed is just a few dollars more than that you can sue in small claims court and ask for $12k and either represent yourself or hire an attorney or you can hire an attorney and sue in one of the higher courts.
Also, the statute of limitations is 4 years for a breach of contract.
If you decide you want to use an attorney, and you can sue for their attorney's fees as well, then the lawyer will want to send a demand letter first. This letter has two purposes, the first to put him on notice that you will be seeking attorney's fees if you have to sue and the second is to word it in such a way that it can be used as evidence at trial.
These are really the only ways to make him pay you.
As to defenses, there's nothing in your facts that raise any particular defense other than the one that is always available "I don't owe the money".
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