If you didn't see him drink for a week, then the vet checked him out and said he wasn't dehydrated, it sounds like he is drinking, just not necessarily while you're watching. That's less scary than not drinking at all, but definitely keep an eye on it. Not drinking anything is more dangerous more quickly than not eating anything, just like with humans.
It sounds like he has a room he stays in when you're gone, away from the other dogs, is this correct? If so, have you tried leaving his food in there (or in his crate) with him for an extended period, rather than putting a 10-20 minute time limit on it? I wonder if having a couple quiet hours alone with no social pressure will let him unwind and be comfortable enough to eat again.
Sometimes a sensitive dog will start getting freaked out by all the fuss made over getting them to eat, and get too stressed by the pressure to eat - weird, I know, but it does happen. Conventional wisdom does say that a healthy dog won't starve itself, but while that may be true for dogs holding out for 'better' food, it sounds to me like there's some kind of anxiety/stress element here. I'm no expert, just a nerd interested in dog behavior and training, so take that for what it's worth, and of course I can't see him in person to make a more accurate judgement, so take this for what it's worth! But it sounds like he might be too stressed to eat without your boyfriend there, rather than him being stubborn or spoiled.
How does he act if your boyfriend is feeding him and you're sitting with them on the floor? Will he refuse to eat if you're too close, or is he fine so long as your boyfriend is present? If he's fine with you when your BF is there, you can start by offering a treat or two while he's eating with your boyfriend supervising, even just dropping them into the bowl so he starts getting used to tasty things coming from you in a scenario where he's comfortable eating them.
I know you already have a trainer on-board, but given how serious this situation is (talk of rehoming, risk to the dog's health, etc), getting an evaluation from a qualified behaviorist might be a good option. A good behaviorist has some training in the science of dog behavior, whether it's a literal degree in animal behavior or a certification through a reputable third party organization like the Certification Council for Professional Dog Trainers or the International Association of Animal Behavior Consultants. A veterinary behaviorist is a fully qualified vet that has further specialized in behavior problems, so that's another option, though like all veterinary specialists they can be fairly pricey. Either way, an experienced, qualified behaviorist is going to focus more on identifying the root cause of a behavior problem, and then design an individualized program that aims to change that root cause. Think of a regular trainer like a schoolteacher - focusing on building skills and manners - and a behaviorist like a therapist - focusing on promoting a healthy mental and emotional state to be better able to put those skills and manners to use.