This happened to a friend recently. She kept calm, ignored his aggressive claims against her, just kept coming back to the best interests of the child. The judge ordered him to return the children to her immediately and put a Prohibited Steps Order in place to stop her ex doing it again.
This won't look good for him at all. I know it's traumatic but truly, he will be back with you and safe soon. You just need to hold your nerve and seek an emergency hearing ASAP.
Do NOT ask for a welfare check and do NOT enter the house and remove the child. Really, don't. He could just do the same back and then any judge would see you as being as bad as one another. You need to be the sane one here, focused on the child. Don't throw mud, don't make accusations, repeat that this is clearly distressing for your son, who needs his home, his mum, and his school, and that any major changes need to be thought through, planned, and agreed as in his best interests.
This man does not have the law on his side. You do. You just need to remember that you're the grownup in the situation.
www.gov.uk/government/publications/form-c100-application-under-the-children-act-1989-for-a-child-arrangements-prohibited-steps-specific-issue-section-8-order-or-to-vary-or-discharge
If you can afford a solicitor, or are eligible for legal aid, fantastic. But my friend wasn't, and got them back just the same.