stand firm on full-time it's his legal entitlement, it does not matter what the statement says, they always have discretion to put in more provision and they have had 6 months to change the statement to ensure it meets need. Basically they are working to a statement which is clearly inadequate - but that doesn't mean that they can keep going with it!
The law says they must meet the special educational needs of the child. They know they are not meeting need and they must put in enough provision now to get him into school.
I am not clear whether there is a formal statutory reassessment process going on or not and a new statement is imminent? If not immediately request an emergency reassessment. At least then you are in a statutory framework. You get a right of appeal either when they issue a new statement or at annual review if they don't change the statement - you need to work out how you can generate a right of appeal
If they can't manage him in his present school they need to look at places that can.
Maybe start a thread asking about independent special schools in / near your area for HFA / AS and go and look.
But I think your original idea of school in the morning and afternoon with a private tutor is reasonable.
Also behaviour is not just an issue for school. Whats the deal here - its ok for him to be challenging out of school but not in school? Do they think his behaviour stops at the school gates?
Have they called in behaviour support? CAMHS? Does he even have a behaviour plan in school designed by a professional?
I can't understand the logic of taking him out of mainstream school completely as a way of getting him back into mainstream school. It sounds as though the school is perhaps not wanting to keep him? They don't seem to be fighting his corner.
I would start making noises about independent special schools - they will not want to pay for one and might start taking you seriously. They will want to try 100% 1:1 in mainstream before they pay for independent.
I would get some legal advice e.g. from IPSEA and figure out where you are in the statement process as you need to get this before a tribunal sooner rather than later. Otherwise I am worried they will be fobbing you off for months.