There's no validity in self diagnosing tbh. What appears as anxiety in someone may actually be ADHD, and what looks lie ADHD in another person may actually be signs of autism. Neither you OR him can say what he has, because you simply don't have the education or qualifications to do so. So it doesn't really matter if he calls it autism or ADHD at this point, since it wouldn;t be a real diagnosis.

What you COULD do is bring up kindly that the symptoms he has listed to you sound like they might be ADHD, too, and ask why he thinks it is autism. That way you are not necessarily saying he is wrong or disagreeing, but just offering another possibility. It is worded as support rather than a contradiction.

Until he can get a real diagnosis, the best you can do is look up the matching symptoms AS WELL AS coping skills and practices for any that he feels hinder him. Someone with ADHD OR autism OR anxiety may benefit from things like breathing exercises, overstimulation avoidance practices, and acceptable self-stimulation aspects like little fidget toys that allow your hands something to focus on and calm the mind. Keep in mind that the stats show there is a big correlation between ADHD and autism- severe or even moderate ADHD can overlap with autism and Aspergers in a way that the symptoms and impacts they have may be difficult to tell apart, based on how that person is affected by them and how severe theirs is.

I think my question would be what exactly is he looking for. Is he really wanting to get tested? Does he need your encouragement for that? Or is he just wanting to give you a heads up in case it affects your relationship?

I've suspected for over a decade now that I have a mild form of ADD but I was never tested. A few years ago I mentioned it to a friend because of concerns it could affect our relationship. She understood exactly where I was coming from and was supportive. Then I mentioned it to a family member who didn't take it seriously and was not supportive. More recently I took an online test for Autism and was shocked to learn that I'm apparently "somewhere on the spectrum". That I've kept to myself, no logical reason I can think of to go there at the moment.

At this point In my life I see no reason to get tested. I've dealt with this, whatever "this" is for a lifetime now. I'm not interested in being medicated or having it on some medical record somewhere. Being diagnosed wouldn't change anything either way, I'm still me whether or not anyone puts a name with whatever causes me to be who I am. On the other hand, maybe if I had been tested about 50 years ago it would have changed something but it's too late in the game to be second guessing things now.

Better to try and support him to get the proper tests and/or diagnosis, then it won't get in the way of your relationship.

If you've met one person with autism then you've met one person with autism. No one experiences it the same. Also ADHD and autism can coexist and they often have conflicting needs so one can mask the other.

I don't think it's fair to argue with him when he is just looking for support.

Get an assessment...instead of guessing.

I told my doctor, I can't remember if he was a psychiatrist or a psychologist, once that I thought I had ADD and he said there was no such thing.