Make sure the parents know what work is involved with the number of children. Explain how you plan on helping them. It's instrumental in the family determining how much they're willing to pay.
A big thing is making sure there isn't something making him uncomfortable. Maybe the teacher yelled at another student, or your son saw something happen. Usually, if a child regresses in that sense, it's because of true fear or comfort issues.
The Care.com Community is an advice-sharing network for parents and caregivers.