Oh, he is eating. He may not be eating what is served but he is eating and drinking. The odds of a 7yo going days on end without eating or drinking are about slim and none.

Test the theory. Lock up all of the food and liquids. He will come knocking on daddy's door within a few hours much less within days. Put locks on the pantry, cabinets, refrigerator, sinks, etc..... Check his room, clothes, bags, etc... for food and drink. If he has them take them.

Once you have proven reality then you know and you can effectively enforce the household rules.

7yos do not get to decide which rules they follow and don't follow. You and DH need to prove this to him with little else considered. This is toxic BM and toxic spawn manipulation pure and simple.

Quit worrying about the distraction of his eating and drinking and focus only on the facts and a logical solution. Once you have the facts, smack the shit out of BM with them and drag her ass in front of a judge. The kid will gain clarity if mommy gets in trouble for her bullshit and his.

Stick to the facts, apply consequences to the toxic blended family opposition and any kids who decide to join the opposition. Lather, rinse, repeat.

I've been thinking about your post and the header...about your ss "not eating for you." You know what? People don't eat for others they eat for themselves. I think you get it, I hope, that this isn't about YOU but about the power struggle between your SO and his son.

I've been through this. My SDs wouldn't eat anything but plain noodles, white rice and toast when I met them. Pretty much their diet between DP and BM (I saw the remains of the school lunches she packed for them) was all carbs.
They refused to eat ANYTHING I made.

So I stopped. I told DP I was making nothing but real food, no multiple types of meals. I would cook one thing for everyone (I did make it so that most meals were things like tacos, spaghetti and meatballs, stir fry - so that one could put together their own food at the table how they liked). If he wanted to do something different then HE could do it.

It worked for the most part.
SDs are much better than they used to be (although SD12 still eats like a 5 year old...ok, maybe a 6 year old now). But they are still extremely picky.

Just be aware your SO will defend them to the end. DP and I just had a discussion last night about how "bad" SD10s friend is with food. I told him yes, she's like SD12. Denials about how bad SD12 is come from his mouth...um, no, DP, SD12 is actually worse...then he's like, well, it's all about the marketing of food. Really? I don't buy brands (except he does for SD12 of course)...and then he says, "You are blaming the parents, why is it always the parents?" Because YOU and BM taught them what to eat, taught them to be wary of new food, to always eat the same thing because it was easier for you...so YES the parents are responsible for the first introductions of food in their early years. NO I am not crazy for thinking it's strange the SD's don't like HOMEMADE ice cream because it's "weird" because it's not the same brand you've been buying them for 3 years now.

OY. Sorry, off the rails there.

Stop trying so hard to feed the kid. Let your SO do it.

My two cents? Don't do the cooking when your SS is over. Have him sit there and watch his dad prepare his meal, have dad sit with him while he eats and when he's done, send him off to his room.

While he's in his room, you and your H can enjoy a pleasant meal. Alone.

Kid is fed. You aren't involved.

H's kid pulled similar BS and she was 12 at the time. I finally had enough one night and dumped her plate of food into the trash and told her to make herself a sandwich and get the hell out of my face. From that day forward, unless we had company or had to eat in a restaurant, I refused to cook when H's kid was over (and at the time he had sole legal custody). I also refused to share the table with her. H was to feed her before I got home from work and then sent to her room. She was not to bother us while we were eating. When she did, because H has no testicles, I would bark at her to go back to her room and not come out until her dad told her we were done.

There are consequences to crappy behavior. Your SS is behaving badly and doesn't deserve to be a part of the family table until he learns better manners.

Quit trying to feed him. Make absolutely no effort for him above and beyond what you do for anyone else in the family. He is enjoying the attention he gets for not eating, and he is enjoying creating strife between the households.

He absolutely is at the least drinking water when you aren't looking. A person can not go days without water. And I would bet there are other things he eats too, just not the meal you're cooking. Your DH can decide if he would like to offer an alternative for the kid to eat or if he is going to push the issue. I personally would make DH have the kid choose between having a meal with family or having his banana with glass of water alone. Then be done--no more discussion about it, no more "Gee I wish you'd eat more", no more "You need to eat what we eat", nothing.