Is it permissible to boycott a grandfather if he has bad manners?

Boycotting a Muslim is impermissible except within a very narrow scope. The grandfather's ruling is the same as the father's in terms of righteousness, so boycotting him is impermissible, no matter his condition. It is obligatory to maintain ties with him to an extent that does not harm the grandson, while advising him, enjoining good upon him, and forbidding evil from him with gentleness and politeness.