That's a tough situation. I've had similar issues in the past, depending on where I was living and with whom. It gets rough sometimes.
Maybe you could add a mental component to it. Like, section off / close off a section of the fridge and have a sign on it that clearly labels it as your children's foods. Then agree that you won't eat it and he won't eat it. Just the kids eat it for school lunches, etc. The idea that some food belongs to someone else has stopped me in the past because it feels unethical to take it. Basically establish that it's not an option to eat it.
If that doesn't work, maybe a separate small fridge that locks. They sell them in stores (my friend had one that he used because his roommate stole his beer). Then hide the key. Depending on the brand and such, a mini fridge with a lock is usually $100-200. Here's an example that's ~$180:
http://www.amazon.com/Compact-Freeze.../dp/B000RHWT3M
Also, he could have ready and easy healthy options for him to grab whenever he feels the urge to eat. Getting really hungry and having nothing prepared is an easy way to overeat.
It's good he's asking for help, though. Good luck!