Obese autistic brother and lack of trust

Hi everyone,

I'm not sure how to navigate this forum so if anyone's had a similar experience, let me know.

My brother is 24 and morbidly obese. He's been overweight since he was a child. He sleeps until gone midday, eats very little during the day, spends most of his time in his room, and when my sister (17) and I go to sleep, he comes back downstairs and binge eats anything and everything he can get his hands on.

We've recently joined a gym together which he's taken to quite well, but he's still sneaking downstairs at night to eat. It wouldn't be so much of an issue if he was considerate to the fact my sister and I need to eat too. He regularly eats more than his fair share of things we buy for the three of us, but will deny it when asked. We're not confrontational when doing so, just genuinely curious/baffled about where things have gone which were literally there the night before.

Recent examples: we did a food shop on Monday and bought 12 packets of popcorn. We're now on Wednesday and he's eaten 5 of them. We also had a jar of chocolate spread which was given to us on Friday. When I came back home on Sunday, 3/4s of the jar was gone, and by Monday, it'd been finished in its entirety. I'd had enough to cover half a slice of toast. My sister had a teaspoon's worth which means he'd eaten the rest himself.

I'm trying to instill some kind of structure to his day i.e making sure he's waking up by 10:30am at the latest, making sure he showers upon waking, making sure he's having breakfast and lunch, involving him in dinner making etc, but I fear he needs more help than I can offer. 

I'm also aware that my family and I aren't particularly well-versed in our approach (I'm 33 and haven't lived with my family since I was 20) so I recognise any of what I've said above is probably coming from a place of ableism but if anyone has any suggestions on how best to navigate this situation, I'd be extremely grateful.