The intergenerational pattern of forgiveness can get complex, as you are seeing. The good news here is that your son need not forgive his grandfather for what he inflicted on you. You need to forgive your father for this. You son should forgive his grandfather for what the grandfather did to your son. So, you can keep the issues of injustice private between your father and you without necessarily sharing the specifics with your son.
I do recommend that you point out the pattern of anger between the generations. This will help your son to see that you and he have learned a pattern of behavior that needs to be broken or else he and his children are likely to continue the unwanted pattern.
Please try to point out the intergenerational pattern of anger to your son in as non-judgmental a manner as possible. In other words, first forgive your father and then discuss the patterns with your son. In this way you are less likely to even subtly condemn your own father as you discuss the anger pattern with your son.