There is so many different things that help for different kids, some that I've seen help are,
If your child is rigid about routines, it may be easier in the long run to skip the potty and go straight to the toilet. Otherwise you have to reteach the skills to transfer it to the real toilet. Try with a kids toilet seat and a high footstool.
You could start doing nappy/pull up changes in the bathroom to try and build the accociation with the baththroom and where toilet stuff happens.
Announcing whenever you are going to the toilet, saying "I need a wee so I'm going to the toilet" etc and if your comfortable you could leave the door open while you are in there sometimes so you are modelling what to do. If your kid is interested in the symbols it sometimes helps if you use them yourself, like while saying you are going to the toilet you tap the toilet sign.
You could start by trying to note down how frequently your child wees. If you can see a pattern to when they go, that would be good times to aim for putting them on the toilet.
If they are not following a pattern, try for 20/30 minutes after a big drink as thats a common time frame.
If he isn't sitting long enough to get all of his wee out, he may need longer to relax. You could try something motivating while he is on the toilet like singing a silly song with him, letting him watch a short video or having some (wipeable) fiddle toys in there that he gets to use only when he is on the toilet.
You could try rewards for going to the toilet (even if he doesn't manage to wee on there) that build up positive associations with going. Always going to be specific to what your child likes e.g. stickers, tokens, a jellybean etc
If the process is upsetting him its okay to wait for a bit, it will be harder to train him if he has built up negative associations, the more fun and light and non stressful you can keep it, the easier it will be to keep him motivated and happy. This is hard especially when its frustrating but in the long run it can help. There is no timeframe for when this has to be done, you are doing really well and it can take years for some children with ASC to fully get toilet training.
The eric website has some good advice.