They can ask you to be his 1:1 while they recruit someone but it is rather cheeky on their part, not best practice, not 'normal', and you can absolutely say no.
It is reasonable for you to want to keep him at home or on a reduced timetable until the school has a plan for meeting his needs. They should be discussing the best way to approach this with you, not threatening to take him off the school roll.
It sounds a bit as if the school are not very well clued up about how to support a child in a situation like your son's, and as if they are flailing around hoping the problem will disappear. This may mean they will not support him well in the long run, or it may just be a bit of clumsiness on their part as they try to work out what to do in a situation that is new to them. I have limited patience for this but I have seen it work out well, personally, so it may be worth reserving judgement.
If they suggested a reduced timetable that was acceptable to you and supported you to apply for an Education and Health needs assessment, that would be the appropriate route.
There will be a SEND helpline for your local authority, probably with a name like SENDIASS. It would be worth calling them first thing tomorrow, describing the situation, and asking for their input.