Check with others in the area what rates they pay,if he has experience in childcare then Id be inclined to pay a bit more.
Remember to tell him where your giong,leave a phone number.Leave him some food cool drink,crisps,coffee etc. Tell him if your child has any favourite toys etc.show him where spare clothes are in case the child is sick.Leave your Drs. phone number in case of accidents.Im sure hell be O.K
As said already introduce him to the child and see how they get on. Then if you feel happy ask him to come round one Sat for a while whilst you go shopping .