Radak (28): That verse is only when the sons continue in them. Then, the fathers' sins are joined to the sons'. This is only for idolatry or other great sins. Hashem promised Yisrael that in the future, their hearts will be complete with Hashem, and the land will be full of knowledge of Hashem, and sinners will diminish. And even the sinners at that time will not sin great sins. Even for them themselves, the punishment will be delayed - "veha'Chotei Ben Me'ah Shanah Yekulal" (Yeshayah 65:20). All the more so a son will not be punished for his father's sins, even if he sins like his father!

Malbim (Yechezkel 18:18-21): A virtuous son should live, even if his father was evil. Just like a father's virtue does not save an evil son, a virtuous son will not be punished due to an evil father. The father himself will suffer for his sins. The father himself, if he fully repents, will not be punished for his previous sins. All the more so a righteous son will not be punished for his father's sins! Physical traits are inherited, but not spiritual matters.