If school say he doesn't need one, then apply for one yourself.
My son had two schools say he didn't need one, and there was no point etc. He now has an EHCP because surprise surprise they could not give him all the support he needed without one. I had to apply for it myself in Year 8, when he was out of school.
My child was not violent out of school though, only a few isolated incidents in school in primary in playground, so your son's needs are either not being met in school fully (ie he is getting through the school day but it is exhausting for him and he is lashing out when safe at home (google masking) or his needs are much greater than school recognises (ie emotional needs and social communication needs)
Parenting skills are the first thing they will attack you over, but if you can consistently use ASD friendly parenting and he is still violent, that is another piece of evidence. Tony Attwood's Aspergers Syndrome and Explosive Child are helpful books. Out of Synch Child is another good one.
Another thing to ask school is whether ds is being seen by an OT at all for strategies to reduce the violence? They could suggest all sorts of calming activities if his trigger is stress related, sensory related. You really need to push for this, it makes a big difference. Hopefully that is part of the therapy they are suggesting for him? If it is play therapy only, it may not be enough.
I found if I told my child off for bad behaviour or used time out in a reproving way it was incredibly destructive and escalated tantrums or refusals, whereas understanding why they flipped (wrong texture food, tired out, toy needed to be in right place, wrong phrase upsetting scary (Ds2's trigger phrase was Coraline 
that was enough to start him screaming, and ds1 unfortunately used to taunt him with this, even though they had never even seen the film)
Soothing activities for ds2 were outdoors, running climbing, nature, dressing up, soft toys with "characters" (one was called Lewis Hamilton) cars to stim with make whooshing noises, long baths with bubbles, and no time pressure, helping him dress, helping him put shoes on, deep pressure, bedtime routine, small tidy bedroom with familiar things all around, no demands (easier said than done) trains trains and football stickers
Triggers were team games losing games wrong food, favourite programme not allowed (best to do a lot of preparation before switching something off in our house, never a punishment) wrong phrase (one of ds2 most hated phrases was This is not appropriate) being left out, unfamiliar routine, waiting in queues, waiting for anything, handwriting, homework which he couldn't do (we scribed everything for him past 6)
HTH