This is a very worrying situation. Children needs stability in their settings.
It is a good thing the Police has been informed and the necessary information logged in their system for future reference.
With regards to the her proposals, if he is not concerned with the proposals that the son is to live with him, he can arrange to see a solicitor on a fix fee basis for that to be drafted into a Consent Order for the seal of the Court. This means that she cannot go back on her word and take the child from his residence (dad).
In any event, if he is afraid for the safety of his family the Police should be notified.
If he decides to apply to the Courts for a Residence Order, the Court determining any question with respect to the upbringing of the child, the Child's welfare and his best interest shall be the Court's paramount consideration.
Also, the Court shall have regards to the wishes and feelings of the child concerned considering his age and understanding.
With regards to the ex attending the family home, there are options available to him such as a Non-Molestation Order or Occupational Order to prevent her from attending the home and harrassing him or pestering him and the family.
Visit your local CAB or get an appointment with a solicitor to explore all options.
Best of luck