Can a contractor charge us more if he miscalculated a job?

Hello! We had a new leach field put into place by a licensed and insured contractor. The contractor first consulted with an engineer to determine the size that was necessary based on the square footage of our house, the number of bedrooms and the number of people who reside here. We agreed on the price, at which time he gave us a verbal 5 year guarantee, and took out a loan to pay him. After paying in full, we saw that the ground was still wet and there were puddles forming in the back yard. After several attempts to contact him, he finally returned and told us the leach field was too small, and he wants to charge us $3,000 more to enlarge the field. Can he charge us more money if he miscalculated the job in the first place? Do corrections/modifications fall under our original contract with him? Does this at least fall under his 5 year guarantee? Thank you so much for your time.