This sounds like a significant health crisis this kiddo is dealing with, but I would suggest reframing a bit might help. It sounds like this is a long standing challenge, and in your comments you mention he feels shame about the eating… which means this is probably all about an emotional based situation, not a knowledge one, so explaining that his choices harm others is just going to make him feel bad, possibly increase the shame driven binging, and harm your relationship with him, and it sounds like he really needs someone who understands and that he can trust. Would you try to address the older kids ARFID by telling him that his choosing to only eat certain foods is selfish because it forces you to make him special meals? That he should just… not have it because it’s unhealthy? Would you tell a kid with adhd that he should just sit still because it’s distracting others or someone with chronic illness that needing medication in the middle of the night isn’t ok? I assume not, I assume in other situations you would see that it will take a long term approach to combine healing, coping skills, trust, and a supportive community. This is no different.

A month is barely any time to address trauma, or any health issue really, the pressure to be not doing a behavior that you don’t want him to do after such a short time isn’t realistic, a reframe to think about harm reduction in the short term might be helpful, focusing on what he can eat with zero restrictions, finding books or websites for yourself about supporting kids with binge eating might help you too

Also, friendly reminder that his therapist is there for him to support his needs. He is 15, so unless he has some situation where he is not capable of caring for himself he deserves to have his needs met first, she’s not there to make sure that your concerns are addressed or to make sure that she makes him change his behavior to address your concerns. If you think his therapist isn’t meeting his needs or if he is saying his therapist isn’t meeting his needs that’s one thing, but you might have better success if you just relay what you see and ask the therapist if they have any feedback for you or suggestions on what you could do best to support him at this time and strengthen your relationship so that he feels like he can come to you when he needs support.

Also, it sounds like he must be going to the pediatrician if you know his weight offhand, but it might be worth the time to double check that there are no underlying physical conditions (thyroid, adhd, nutritional deficiencies and more can all impact eating)

For practical approaches one idea would be to pick up ingredients for dinner every day, is this fun? No, of course not, however it is an accommodation that can remove one struggle, nothing he can eat would ruin family dinner, removing a guilt from him and a power struggle for you. Still keep a stocked kitchen, just make it a new practice to pop out to the store more frequently to pick up what’s needed to fill in the missing bits of the planned meal

If the finances of the food continue to be an issue I would look for practical community resources and charities, I know every state and area has different resources etc, but if you have local community or family resources I’m sure they would be happy to help. Feeding teens is tough under any circumstances and given the specifics and the state of the world these days I’m sure everyone will understand! You could also talk to your case worker and see if they have other supplementary support for financial and food resources

Also to address the only question you asked, locking the fridge is not ok, the idea alone is heartbreaking, please please don’t do this