J started school with no dx of anything except tentative ADHD dx - had vvvvv challenging behaviour and was awarded 23 hours after initially being told he wouldn't qualify and then applying to Tribunal - LEA backed down. Was told I was lucky to get 23 - didn't feel very lucky, given that the nursery and I had said he needed 32 for health and safety reasons, but luckily the school upped it to 32 from their own budget (but didnt have to - was just lucky that they were co-operative).
J was then dx'd with ADHD and then subsequently with autism too and my repeated request that the Statement reflected the hours that he needed was finally accepted (after again applying to Tribunal and writing to MP).
He is now 7 and gets 33 hours of 1:1 in school and the school also get 5 hours of funded liaison time for all the liaison with outside agencies. Not that it will be used for that, but there we go.
IME, you have to push and push for the right amount of time. It seems to be harder if the child's behaviour is good BUT you ds has additional physical needs so I would push for full time if you think it is needed as you would have a good case, also with him being non-verbal.
The first stage is to refuse the proposed statement if you're still in the consultation stage. Ask for a meeting and set out why he needs full time. If they issue the final statement with too few hours, you can appeal to Tribunal. Your MP is also a good route - mine was excellent and always wrote to council with strong letter.
If you've already accepted the Statement and it has been finalised within the last few weeks, you can still appeal.
If it's past that, you may need to ask for a review and ask for amendments, but they don't have to agree and you may have to wait until the school ask for one or for more hours.
LEAs don't like Tribunals because they usually lose them so very rarely go that far if they're not likely to win them.
Ignore the 'lucky' comments - we got that. No, we are not f-ing lucky. We don't want our kids to have SN and these hours only go some way to make up for the fact that our children can't access school like other kids. If they say lucky, they mean lucky because the system's so broke that very few kids get what they need. But that's not your problem. Your ds needs the right amount of support for HIM and HIS needs, not in comparison to some other, unnamed, possibly underfunded children.
Be strong - good luck.