Actually I think a lot has changed, you have way bigger schools than ever that effects cohesion and also the amount of attention children get. Even kids who are struggling can't get as much help as they should and then it gets so bad and then they are top of the list. More schools have year groups of 200-300+ with one head of year so obviously kids are ignored.
Also with MATs less teachers and SLTs even have 'control' of their schools the policies, curriculums they know work for the schools demographic and kids, there often isn't that personal level of situational judgement. The parent, teacher, pupil school breakdown for which all sides are responsible has gotten further polarised because of the smallish amount of individuals in those groups who treat the other badly. There's no trust between such groups.
Way more arbitrary rules on things like appearance, not turning up with a calculator or P.E kit (not taking about repeatdly but a kid having a stressed day forgets something for the first time etc or sometimes a kid whose shoes are broken so they have to wear trainers, and their parents genuinely can't buy another pair for 2/3 weeks etc). Those things don't help or grow students, they just overly penalise them and also hurt their own relationship with education etc. Exams and GCSEs, A-levels are harder than ever, no youth clubs or very little, no sure start etc, little help from YOI or youth workers if needed, lack of access to sports and enrichment due to cost, bed poverty, clothes poverty is much worse, and many teachers etc/schools fail to have culturally compotent policies and policies which acknowledge this, many don't even think of x reasons because they haven't been near that ever. I'd say that schools yes have gotten better in some way but in many ways much worse, some not their fault but other are. If I look back at my school experience not that long ago, I'd say honestly the amount of racism just from staff was traumatising, in professions like teaching choosing ignorance hurts students, choosing overly arbitrary policies eith little situational judgement hurts both staff and students. Obviously I could be wrong, your experience is yours and I wasn't alive then but just my thoughts, also the thoughts I've heard from lots of students, both anecdotally but also through some of my longitudinal research etc looking at such changes or whether people there is said change.
Back to the main point OP, is there any way you can get him school therapy or therapy outside of school. It may help to have a place to decompress etc. Look it something like somatic approaches, etc. Does your school have a peer mentor programme? They can be amazing for many kids. Can I ask @HelloKittyFan what adjustments in primary school he had?
Did the primary school definitely refer him to CAHMS, for a psychiatrist to assess him? Did you do any forms etc?
Maybe through the NHS app, check hid medical records, see if any letters or notes have been added etc. I'd check back in with the primary school, to see if they did the whole thing, as often you ask and they forgot to send one bit off etc, or perhaps though the secondary SENCO would pick it up, even just ask for the specific service.
Does the webiste have the direct emails of any SLT, particulary anyone for DSL/Pastoral, or KS3 that you could contact? Or are you able to go to reception in the morning/early before school/form time in the next few days? Or afterschool instead?
Really be persistent I know you've tried, and tbh you shouldn't have even had to do as much as you have, but the system is broken and only parents who fight and advocate and work around it get results. For parents who are EAL, deprived, work 2 plus jobs etc, they often have no clue about it and are failed, but you seem to have access to resources so game/fight the system.