It isn't easy to know whether it would have changed anything, if you had got the diagnosis earlier.
Maybe it would have encouraged you to fight harder for more accommodations when he was in sixth form/doing A levels, but sadly this seems to be dependant on what the school believes, with or without a diagnosis.
As my DD hit crisis aged 11, at the first half term of Y6, she was already diagnosed autistic. Yet when I had asked repeatedly for more support, emailed school repeatedly telling them how awful the after school restraint collapse was and how bad things were for her, I was constantly reassured that she was "fine in school," with a huge implication it was a home problem and was told she did not need and would not get any further support.
Once she hit crisis and they "saw" it, my rule following DD absconding by climbing the gate one lunchtime, running away in the mornings so we ended up chasing her down the local roads, not able to do any work, on edge constantly, in tears and begging to go home sometimes, THEN they offered a lot more support. But it was too little, too late. My DD has a formal diagnosis of school trauma from the CAMHS psychiatrist now.
And I tried, I really tried to get her more support. When I appied for her EHCP, I asked for the school CPOMS (their incident/safeguarding recording system) for her and saw they had watched a webninar I sent them on autistic girls in my desperation (it was recorded her teacher and the SENCO had watched it), saw my emails about how bad things were at home, snd how they had recorded, "Mum says the family are desperate." Even that makes me feel guilty, because I could have taken her out sooner. I should have. She is still very isolated, with no peer contact at all, 4 years on. She doesnt chat to me much at home, but her mask is super chatty and smiley out of the house. She'll talk to anyone, but had never managed to make a friend (as she'll pretty much only talk about her things).
It sounds like your DS may have had a period of burnout, or near burnout, while doing his A levels. It may be that he needs this time of not doing much else to recover, and then could explore what might suit him. Groups around hobbies or interests might be helpful, and so many say that knowing you are autistic can help with being kind to yourself and understanding why you find certain things so difficult so if he is keen, he could request a referral for assessment now.