Have you ever tried to get services involved? I have. My teen had severe mental health challenges, also autistic, dropped out of education. GP referral led to an 18 month wait list for 'therapeutic support'. By the time that came round, they had taken an overdose and referred to CAMHS. 6 months waiting for an assessment, by which time they were self harming. 6 months on from that and still no CAMHS intervention.
Meanwhile, we had referrals to Early Help from me, school and GP. LA said they couldn't help. It took a mental health nurse to get involved and say there was violence towards me from my child and I was at risk of carer exhaustion before additional services were involved. 6 months from that and me begging for help, emailing all services in a shared email saying I couldn't cope and I was genuinely scared for my child's safety, my safety and that of others before a social work assessment was done.
Even now, the "involvement" is a visit every 3 weeks from the social worker and a phone number for CAMHS duty (Mon - Fri 9am - 5pm, leave a message and you might get a call back after a week).
At one point, my child had intrusive thoughts about harming others. I relayed this to CAMHS who relayed it to another agency involved in preventing violence. After two weeks, it came back as "low risk" but I should always accompany my 17yo when they left the house (ignoring the fact i had a job outside the home), hide the household knives and anything else sharp (i locked them in the car boot overnight but not exactly practical during the day) and call the police if my child threatened or hit me.
I was ACTIVELY trying to get help, from all services, on multiple occasions. It is not easy. There isn't someone on hand 24/7 to give advice. You feel incredibly isolated, like no-one is really listening to you or trying to help.
I can totally understand how his parents were acting to protect themselves and their other son, believing the risk to be within the family and that they were attempting to manage autistic behaviour that challenges with limited support (here, attend this parenting course FFS).