Parents should not intervene when faced with their child being bullied in school.
Teach the child to fend and stand up for him self. They need to learn it.
Teach him all the avenues for recourse from, complaining to the school management, the bully's parents right up to making police reports.
Also teach him to fend off physical attacks. Martial arts training is good.
Lastly let him know if all other recourse fails, let him know you support physical retaliation.
And if the bullying involves a big group, support him in armed response. Teach him where are the carotid, femoral and brachial arteries are.