You must take a firm line. When a child is excluded, the responsibility is thrust directly on the parents. This attempted abdicaton of duty is unacceptable.
If the boy is too young to be sent home to look after himself, the case should be referred to the local authority's education welfare service, which will visit the parents and make the position clear.
Technically, the boy is a trespasser on school premises and could, of course, be removed, if he failed to leave voluntarily.