Is it permissible for a mother, according to Sharia, to retract her forgiveness of her son's disobedience without a reason, and would the initial forgiveness be nullified despite the son's eagerness not to give her an opportunity for that?

The mother's right is great; she is the one who bore, stayed up, and toiled for her child. Abusing her verbally or physically is an act of disobedience and a major sin. What is incumbent upon the one who commits such an act is sincere repentance to Allah. If he repents sincerely and his mother forgives him, then there is no accountability upon him, for repentance erases sins, and "the one who repents from sin is like one who has committed no sin."