I don't know how much help my reply will be, I'm really just replying because I feel for you and wanted to show a little love and support :)
Hopefully it'll be easier when you move next month. My only ideas are maybe looking into any nutrition classes for kids in your area? Then he'll be learning about food and being healthy from someone else, not just mom.
My daughter was about 10-11 when I first joined Spark and started eating healthier and cooking healthier. At first she sort of rejected it- she'd eat the healthy dinner I'd just cooked but want to have a big dessert after, and she started asking for candy and fast food way more then she had before. I think it was a fear of something new, thinking she was never gonna get those things again so she better over-do it while she could! what I did was just anytime I could naturally work it into the conversation i'd talk about my reasons for wanting to eat better and be healthier (like wanting to be able to play longer with her, wanting to live longer, etc.) and I'd thank her for being on my team and supporting me by trying to eat healthy too. How when she made good choices it was easier for me to make good choices cuz we were doing it together. I think that helped with her. She's 15 now and eats super healthy and is super active even though she doesn't play sports anymore, she jogs with her friends and walks shelter dogs almost 3 hours a day.
Does he play any sports? I'd definitely keep him active and playing while you are trying to get his nutrition in order. If he doesn't play sports I'd try to find activities to keep him exercising like going to the park or riding bikes.
Maybe he's in a growth spurt right now that would make him wake up at night hungry? I'd tell him if he wakes up at night hungry to come get you so you can find him something to eat- maybe some crackers with peanut butter and an apple, or toast and a banana if he wakes up hungry. Maybe he thinks you won't let him eat in the middle of the night and that's why he sneaks out of bed to eat? Maybe if you let him know it's okay, you just need to know about it he won't eat so much at night?
Do you think it might have anything to do with him being worried about you husband when he's gone? I work with a gal who's daughter and husband are both Army and occasionally she has the kids for a few months when they're both over seas at the same time. The kids always gain weight with her and she thinks it's a comfort thing, they want McDonalds every day and ice cream every night. But once mom&dad are back they lose the weight. They think it's the stress of worrying about them when they're both gone. It helped a little when she stopped letting them watch the news, they hear all these bad things on the news and don't really understand that what they're seeing isn't even close to where their parents are.
I really hope you find a solution soon!