My overweight (16st) adult son would eat 5-7 sausages with the other food you describe if I left it out cooked and said 'help yourself,' but he's now aware he has an eating disorder. He is losing weight just by having lots of extra veg and no potatoes, rice, bread, cakes, biscuits, crisps or sweets, and going for a half-hour walk each day (NOT to the chip shop!) Perhaps your DH could try that to start with?
I disagree with PPs who think you should divorce your DH for eating 5 sausages (FFS!) A fat person needs to eat more to maintain weight than a thin person. Yes, ideally he should be trying to lose weight, not maintain it, but unless he has decided to stick to a reduced diet, giving him smaller meals will only mean he fills up on expensive snacks, which cost more and are less nutritious.
Taking your special snacks, or snacks reserved for other family members' lunches, is bang out of order and if done deliberately, should result in a massive row, followed by him apologising profusely for being so selfish and immediately going out to buy some more to replace them. If this is not what happens, maybe you should be asking yourself why you and he are acting as if he has more rights to nice treats than the rest of the family. How fucking dare he!
Yes, he could buy a lockable box for you if he feels he can't control himself, but you shouldn't have to buy one to defend food that isn't for him, you should only need to make it clear which snacks are for him and which are not, so he doesn't eat them by mistake. If he's eaten his share and wants more - tough, make some beans on toast instead.