OP, put this in writing to the GP.
Explain when it started, the paranoid beliefs and his change in behaviour. State the potential risks. Write about your interaction with the crisis team. Mention any change in sleep and appetite.
Say that you cannot wait till the 20th. And that you have a child at home.
As it sounds like he still has some insight, your husband may be able to work with the crisis team and avoid an admission if he gets an assessment and meds asap. It is harder to ignore the written word.
Give this letter to the GP receptionists tomorrow first thing and say it’s urgent and you would like the duty doctor to see it asap.
Good luck.
And as others have said, you are either an informal/voluntary patient or a detained patient sectioned under the Mental Health Act. The numbers of the sections eg S2, S3, S136, S5(2) refer to the relevant section of the act which details the conditions for detention and appeal. A voluntary patient has nothing to do with the MHA, sections or detentions, and has capacity to consent to an informal admission.