REFUSE
Whatever you do refuse absolutely to agree to this proposed placement in a (former) PRU.
I have a lot of experience of this - been through it.
The small unit (that is not a PRU) will be a centrally run facility funded by the LA directly. It's sole aim is to get your child straight back into the same mainstream placement that he can't cope with at the moment, as soon as possible. It will do nothing to address the reasons why he cannot cope in his current mainstream school.
They'll tell you it's not a PRU - but it is. They'll taell you that it's a small unit designed to provide additional help and support blah blah..... but it's just a holding pren while your DS and his school have a break from each other for a short period.
What you need to know (and to tell the LA) is that PRUs (and these small units) are not designed for a child with long term special needs - and because your child has a Statement he obviously has long-term special needs. Of course the LA know already that this is not a suitable placemnet for a child with long term special needs as the Dept of Education has reminded them of that fact - so the LA are just trying it on to save themselves the high cost of providiing the suitable support your DS requires. (Just like they tried it on with my son too).
You need to reject this suggestion very firmly and remind them of their statutory obligation to provide a placement that is suitable to your child's age and ability that he can access - as he can't access his current school.
Then you need to look hard at what he needs to be able to remain at a mainstream school and whether his existing school can provide that support or whether you need to start looking at alternative mainstream schools or independant placements. It may be that his Statement needs to be reviewed to identfy additional support.
The last thinh he needs is to be shuffled off to a 5 week placement where these behavioural 'experts' who haven't a clue about ASD will not improve matters - and then be shunted back to the same mainstream provision as before. That would just make matters worse.
So say No and you're within your rights to do so. Ask for an Ed Pysch assessment to identify the problems he is facing in mainstream and start looking around for a school yourself that can support his needs and don't limit yourself to those funded by the LA.
Once the LA know that you know that what they are suggesting is not suitable for a child with longterm special needs so should never be offered in the first place - you can start to talk to them about appropraite support.