Remember its not his fault.
Id imagine at 9 with autism and already on his 3rd "mum" its a lot of change and incredibly unsettling. He cant self regulate his emotions at this stage which is why it is coming our like this.
You definitely need help, have you looked st the autism charities. Also is the lcoal autism service running a parenting course foe those who are parenting autistic children. I think its completely different strategues when your child is on the spectrum.
How about at school? Sometimes a clued up SENDCO or pastoral worker can have ideas or links to support if not.
Certainly access all the help you can.
I found it helpful (my child isnt as violent but is autistic) to learn that meltdowns arent intentiinal. When the child is "losing it" its because they feel out of control, and by that stage its too late. They are often exhausted afterwards and worn out and need looking after not telling off.
Have you looked for triggers? You can use an Abc sheet to see if you can spot any patterns. Sometimes you can't, but it maybe they cant cope with certain clothes, too many instructions or busy crowds or shopping and noticing it helps. Sorry if you've already done this.
Do you keept hings simple at home. Have a routine. Keep bedtimes similar. Do you have a visual timetable? That was v v helpful to my child. Shes ever so bright but couldnt sequence in order and found she got overwhelmed v quickly and that helped
Have you seen the coke can analogy? Google coke can analigy and autism - it can show you all the little triggers that can overwhelm a child wih autism before they explode at the end of the day.
I think it can be incredibly hard work. Are there any sen support groups near you? Theres a youth club here which is aimed at those with sen and seems fab.
Sorry if its all obvious or unhelpful. Each child is v different but Im aware of the overwhelming feeling that its too hard. These are things weve found helped.