Ok, so this is not the way it works. At all.
What should happen, is that each LA has a pot of money, to act as 'inclusion funding'. The aim is to allow a child to be included in a setting which may otherwise be unable to cope with them, or would lack the resources. That may mean staffing, or equipment, or learning aids, etc.
They should be asking their version of an inclusion officer, to assess your DS' need for 'inclusion funding'. This doesn't happen overnight. But should happen within a week or two of him starting. Then, the assessor will submit paperwork, to advocate Inclusion funding. Clever Inclusion Officers, will say to a setting 'start doing whatever you have to do now, and we'll reimburse you a backdated amount when the paperwork goes through. That is especially true if it is unsafe for the child to be without 1:1, for example.
The trouble is, that it very much depends on what grounds the child needs 1:1. Anxiety..well the child may be anxious, but no physical harm will come. Lashing out - must take action to avoid injury.
I'm not being harsh (and if I remember correctly, you said that your DS is capable of lashing out?), I am just trying to explain, that educational settings will first think of 'risk'. Something can have high risk of happening, but low impact. So that will be seen as an acceptable risk. Likewise something that is low risk of happening, but would be very high impact, would be seen as an acceptable risk. High risk, high impact...well there's a problem. You see?
My personal feeling is that you need to say that you want him to attend his 15 hours, and that you would like them to give him additional support while he is there.
I have to say, again, that it very much depends on the setting. DD1, in the preschool she was in, could NOT attend without 1:1 at all times. Just wasn't safe. At school, now, she is in a class of 10, with 5 staff. She doesn't need 1:1 because she has the right environment.
What I am saying, is that 1:1 isn't the be all and end all. The right support is. Sometimes, the right support IS 1:1...but not always, and no amount of 1:1 will make up for the wrong sort of support.