It will be a question of fact for the jury to decide whether his contest has merit. Your point about who drafted it is an argument you can make to call the challenge into question.
Since this is a Pennsylvania dispute, I can't offer legal advice about it. I'm licensed in California. If neither of you has signed the documents, it seems a fair guess that it's not worth the paper it's printed on. Here in CA, if the agreement was valid when signed by both parties, it's difficult (though not utterly impossible) to later invalidate it. You'll want to get sound advice from an experience attorney in your area. This is NOT a matter I'd recommend any litigant try to approach without representation or, at the very least, advice of competent counsel.
Paul Staley practices Family Law and Bankruptcy in San Diego, California. He can be found at http://www.sandiegofamilylawyer.com