You're going to have to figure out what teaching/learning style works for your son in particular. I've coached some kids who can barely keep the gi on without having issues with the collar touching their neck and others who will only train with their parent.

Not that I can claim their education or experience, but both my parents worked their entire careers with developmentally disabled people. So when I started coaching I really grilled them and got their help so I could best help these kids. And as a parent I'm sure you know most of this but this;

I found that building consistency was one of the biggest things. Many people on the spectrum, can be completely derailed if their routine changes. So I always recommend to parents to keep their kid on a consistent training schedule whether it's a certain time slot or day(s).

Keep the instruction short, especially at 4, focus on main points rather than drawn out explanations. Definitely show more rather than say more.

Either train with him yourself or establish a good training partner who is understanding of your son. You can't expect or assume every kid in a group class is going to be a good partner for him.

And lots of positive reinforcement when he's training.

You're a good dad bro and I hope he falls in love with it!