I'm sorry but I disagree with your perception that you've done as much work as you can around your son's separation anxiety. He's still young, so he can still learn. Your partner shouldn't be held prisoner in his own home, b/c you allow your son's rigidity to dictate both of your lives. Who does that help exactly? I'm not attacking you because I empathize with how helpless you both probably feel as adults with an autistic child. But you can't let your son dictate your lives. You have to be in control here. Not him.

Have you applied for a service dog for your son? A service dog can help him cope with separation anxiety.

I'm just saying, there are resources out there -- no matter what country you live in -- for autistic support. It's just a matter of finding a way to get those resources that is the problem b/c its time-consuming, wading through loopholes, exceptions, and everything.

You need to work on your son's separation anxiety with your partner and figure out how to do that, or your partner will forever be a prisoner in your home and how exactly is that fair to him? I typed "techniques to stop separation anxiety in autism" and a multitude of websites populated. So, you have resources online you can try. But you have to try. Not trying, is the worst thing you can do for your son, for your partner, and most of all for yourself and the health of your family overall. Your son needs to be taught coping skills with regards to separating from your partner. Whether its by you, your partner, or a third party. But those skills will help him later in his adult life, whatever that looks like.

My nephew is almost 6ft 4 and he is barely verbal with his autism. But, he has been taught how to cope with separation from his siblings and his parents through verbal cues and different activities. For example, maybe giving your son gum to chew will calm him down. Maybe playing hide and seek with your partner will teach him about separation and that there is always a positive outcome. My point is, not to beat you up, but to urge you to open yourself up to exploring ALL the different methods that exist -- whether they come from autistic experts, or other parents of autistic children. You have to try something. Change is inevitable in life. Especially with autism.