I include the son since he stands in his father’s stead to be מייעד her and for an inheritance field - מרבה אני את הבן שכן קם תחת אביו ליעדה ולשדה אחוזה
Summary:
The גמרא cites advantages that only the son has like יעוד ומקדיש שדה אחוזה.
Rather it is it is Mederabonon; A decree, perhaps he will revert to his corruption - אלא מדרבנן גזירה שמא יחזור לסורו כולי
Summary:
The רבנן allowed the גר to inherit [only permissible items, not ע"ז], because they were concerned that they גר may mistakenly assume that if he is חוזר לסורו, he will be able to inherit his father (מדין תורה).