The boy was incorrigible and a constant source of trouble to his mother.

Truant : a pupil who stays away from school without leave or explanation.
Inveterate : having a particular habit, activity, or interest that is long-established and unlikely to change.
Rash : acting or done without careful consideration of the possible consequences; impetuous.
Dishonest : behaving or prone to behave in an untrustworthy, deceitful, or insincere way.
Incorrigible : (of a person or their behaviour) not able to be changed or reformed.

Synonym of Incorrigible is Inveterate