You do need to be very careful here because it is a criminal offence to obstruct an AMHP in the course of their duty so whatever you do, you need to ensure you don’t open yourself up to arrest by the police.
If you decide to literally not open the door, the AMHP can obtain a warrant from a Magistrate which would allow the police to force entry in order to take your son to hospital.
Talk to the AMHP or NHS staff - try to ensure they’ve heard your concerns and ask them to listen. But ultimately, s2 MHA is a legal process so it’s backed by various powers for the police to become involved.
I hope it’s resolved and any unpleasantness avoided. Remember: once your son is in hospital other rights and protections kick in, not just application for discharge but a MH tribunal as well.