Partner shutdown

Hi, I’m new here and I’d like your opinion in my case. My partner (M39) has high functioning ASD, we dated for 4 years and lived together for almost 2. 
Sometimes in those two years I reckon he didn’t feel quite clicking with the idea of me in his life, sharing his time. I asked him over and over if he was happy and his response was always yes. 
He has several typical characteristics... direct, sometimes cold a and distant, very quiet... which I sometimes struggled but that happens in every relationship. The only times we argued ... were 4 times (Communication related)  in these 6 years, we survived the 6 month lockdown pretty much living on top of each other and other issues. 

In Jan this year, we got our own place, he was delighted, decorating making the place our own etc, big plans for the future, start a family.  

Then, 2 months later, we had a social with his bridge friends ( that is his thing) and I complaint to him that, he looked so happy and I felt bad because he was not like that with me. That was his last straw, he told me he wasn’t fit for a relationship and we needed to end it, he didn’t see a future and he rather be alone, overnight after 6 year and with no afterthought he bailed. 

Im in shock, his life continued as if I never existed, I tried to contact him several times to make things work but he is Closed in the idea that this is over. 

I don’t know what to think, it feels like he took the decision on a whim, he is being making nonsensical decisions after that, he won’t talk to his family or do things that align with what he told me. 

It feels more like a meltdown, he says he loves me but he needs to be alone, I reckon that if he doesn’t  love me, well... that’s ok. But it worries me that he is having issues that may affect his mental stability and he doesn’t know he needs help. 

Sorry for the long post... has anyone had an experience like that?