There are no formal qualifications to be a babysitter, as long as you and your son is happy with him then that shoud be enough.
As for pay, then that you will have to decide between you and him (min wage £5.35 a hour)
if he has done the Cache then he must have some experience of children.
You could always ask him to look after him one day while you go to the shops as a kinda 'test run' to see how your sone gets on with him.
I'm sure he will be fine.