@Bananarama77 this might work.
So weird approach but bare with me - can you take him to a place that does range of different bed sheets (and ask him to touch different fabrics) and see which one he likes the best and then buy it? Then explain that you need to get x amount one for x amount of days, the other for y amount of days and whatever day you change the sheets will be "Monday" and always do it on a Monday.
It sounds daft but allowing him to chose the sheets and telling him on x day sheets get changed will help him order it if it makes sense. You can even get him involved in doing that.
I'm generalising here but asd people like structure and dislike change.
I'm no expert but it's sounding v sensory issues.
Also I know you said he's climbing into bed with you, you need to explain that no one gets a good night sleep like that and he needs to sleep in his bed so he's well rested ext. Then I would ask him what would help, my recommendation calm app (stories for bedtime) glow in dark stickers, also weighted blanket, white noise. Routine will help calm him down prepare to be by the minute.Get him involved in choosing saying I saw this x thing would you like this or that. The this or that approach is easier to process as it's not looking at all the options. Watch out for labels, scratchy surfaces or things with strong scents. Noise cancelling headphones for loud busy places.
Many people with asd do present younger than their years (iMO please people don't lynch me) usually because there's a v surface level of understanding and they get anxious when they don't understand and may not have the words to say what's wrong. You would be doing a kindness to explain things that you might over look. Ie when you yell I can't hear you so it means I can't help you. Literally point to your ears.
All communication even melt downs is communication.
It could be anxiety as someone else pointed out. The above would help with that too because often giving a choice of x or y simplifies the noise and let's him have a safe element of control without jumping over boundaries.
It's a lot. I really get it there a lot of ND in my house also. I'm rather lucky because I know what will set the kids off so I can usually head it off at the pass. Btw if it is ND - chances are there's a genetic link somewhere in the family tree.
Look out for the words - character, offbeat and quirky to describe relations. And don't panic to much about the future for him. If you can get the structure in place and help him regulate it will do him wonders. My asd didn't hold me back (I realise it's a range and I'm not saying it's the same for all) but I have done pretty dammed well for myself career and life wise. But the key was structure and luckily a few "quirky" family members leading the way.
You got this.