Priory Lodge? It's a boarding school in Hampshire for HFA/Aspergers boys - so he'd still be able to access the full national curriculum inc 3 sciences etc but would have expert specialist behavioral experts on hand. Only affordable via statement/tribunal so giird your loins ; )
You'd have to fight HARD for it but seems a much better option than a PRU.
A managed move to another asd unit? Facilliated by failures to meet his current statement/needs? Sit down objectively was the school fulfilling the statement, if so did that statement accurately reflect your child's needs or has it just been lazily rubber stamped at each annual review for a few years while your sons needs for support have in reality increased? (Noone's fault just how it goes sometimes iykwim).
If so ask for an URGENT statement review including a proper reassessmnent of your child's needs as part of the managed move process, so that the next placement has everything in place it should to support your son. If you can afford it avail yourself of a private report from someone like Linda Miller to facillitate the process.
IF a PRU is suggested - go and look at it before passing judgement, the unit my own son spent a couple of months in was staffed by a teacher who had a son on the spectrum - he learnt more there than in 2.5 years previous in mainstream within the 1st month. Staff of 4, kids = 4. Perfect after a bad school experience and I'll always be grateful to that unit for what they did for my lad when he was at his lowest point. I think of it as 2 months free ABA and more on the lea tab lol! PRU's vary massively from holding pens for the feral (and frankly savage) to what my lad experienced depending on the set up and authority. If the one you are shown (and you may not be!) is a holding pen go legal and fight. Home education is a legal option to you and education otherwise will be able to help.
Do remember this happens to 1 in 8 asd kids according to the NAS so your son isn't unique (i.e don't waste energy feeling ashamed, focus on the road ahead - sadly yours is a common experience). As well as IPSEA the NAS has a great database of people who may be in a position to help and advise you locally.
In order to permanentely exclude rather facillitate a managed move the school have to prove no disability laws have been broken. IPSEA can help here. If you feel you need to go legal the others here will have great suggestions for representatives, from hotshots to advocates if you can't aford a solicitor right now.