Good job!! I think you did exactly the right thing!! If he doesn't know that now - He will later. BTW - Where is Sweetwater? I'm in west Tennessee. I've heard of Sweetwater but can't remember where it is.
One more thing and then I'll stop posting- Well no promises--lol
This may one of the times that will come up later in life with your son- One day he may be trying to teach a lesson like this to his own kids - Then it will be like - One time I made a deal with a guy for my chickens...
I'd say make him keep his bargain- But I would call the guy and tell him the situation - so he won't feel guilty if your son balks or says something to make him feel bad. I would hate to go pick up some chickens and thought I had broke some kids heart!! LOL