Is it forbidden to doubt that one's mother practices sorcery on her son, and what should the son do in this case?

Accusing a mother of sorcery without proof is an abominable and forbidden act, for the default assumption about a Muslim is innocence, and one must have a good opinion of Muslims, and a mother is even more deserving of that. Righteousness towards parents is among the most obligatory duties, and disobedience to them is among the greatest sins. You must repent, think well of your mother, and treat her with righteousness. Maintaining regular remembrance of Allah (dhikr) and [reciting] ruqyah (definition) protects against and treats sorcery.