Contract law is interesting here. There's something called implied consent that makes the previous events very relevant - "Implied acceptance occurs when the parties act in a way that indicates their agreement with the contract terms. It also occurs when a product or service is purchased; payment for the item in question is considered implied acceptance.". This can be verbal or nonverbal.

So since he acted in a way that indicated acceptance previously, the form of acceptance (single text, positive reaction) it's implied he was accepting. A hand shake also constitutes acceptance if it was face to face.