Put a large lock on the cupboard door. No explanation. Give you and kids the key and advise them to lock immediately after accessing.
itll make a strong point that you don’t trust him but hey, he isn’t to be trusted
this is about a lack of respect for other people’s wants and pleasures, but also a drive that is stronger than the “shame” of upsetting his loved ones.. He is prioritising his , not entirely unreasonable, desire to partake of some of the food/drink items, above the fact that it will upset you and kids and deprive you and kids of things your looking forward to. And we can’t always have what we want at expense of others.
im not convinced it just selfishness. It sounds like an unmanagmable compulsion, an addiction to the items he sees and wants, and no ability to control that urge irrespective of the consequences.
instead of telling him he’s being selfish, treat it like a addiction and mental health problem with him, say that explicitly “ clearly, despite me explaining why this upsets us, you cannot control yourself and this compulsion is damaging your relationships in this aspect, so I’m locking the cupboard to help you control it”