Is it permissible for a son to be frank with his father about the psychological pain he suffers as a result of his father's pressures, and is it permissible for him to emigrate with his family to another country to avoid these pressures, while fearing sin for separating his father from him and his grandchildren?

Obedience to parents is obligatory in matters that do not involve disobedience (to Allah), even if they are فاسقين (defiantly disobedient/sinful). This obedience applies to matters that benefit them and do not harm the child. If a matter is difficult for the child but does not harm him, then obedience is obligatory; otherwise, it is not. Obedience to the father is not obligatory in matters that harm the child in his religion or worldly affairs. It is permissible for the son to travel if it fulfills a need for himself or his family, such that if he were to forgo it, he would be harmed, in accordance with the hadith: "There should be no harm or reciprocating harm." Travel does not prevent showing dutifulness to parents and maintaining kinship ties through calls, messages, gifts, and the like.