The son should never forgive his birth mother. When his mother was happy, she never thought about his son's extremely difficult situations. Now, she was abandoned by her boyfriend. Her son has overcome all the difficulties. She wants her son back. The son should never recognize this unresponsible birth mother. She only wants to enjoy happiness without effort.
To hate someone is worse than to love someone because every time you think about this person it will spoil your day and life. The best thing to do is to be friends with that person and then both of you can live in peace, life is too short to be unhappy.