Mc was already 18 years old, so it's his 19 birthday, and in both cases he would have been informed by the lawyer before that, i think that would be just lazy writing.
If Dr Pinkcake want to make it better he can make it so that the mother faked her death because of her family pressure and payed the hospital to lie to her husband about her death, so she is still alive.
Or he could make it that his family from the mother side, want to reconnect with him (grandparents, aunts and uncle, cousins) and from that he start a relationship and they can give him a part of the family wealth.
An inheritance from nowhere would be lazy writing.