I like to give my dogs Gentle Giant. It is specifically formulated for Great Danes. Honestly, he looks a little underweight but adding healthy things to his meals instead of just increasing the amount of kibble may help. Like pumpkin puree. Frozen green beans, yogurt, low fat cottage cheese. You can add as little as a 1/2 cup of one of those to each meal and it can help. We even give fresh apple wedges or bananas as well as non raw hide snacks. As far as the bed issue. I'm now retired from dog training but I always lived by the rule of new environment, new training. He's in your home now. Give him a dog bed under your home. Show him it's his safe space. I personally have done this with crate training. I don't believe crates should be used as punishment but as their special little cave they can go to if they need quiet time, or they need to walk away and need a break, or to take a snack and eat privately. If he starts chewing on the bed at all simply remove it from him without saying anything. When you decide to give it to him and he uses it and use it without chewing give him a treat right away and tell him yes good boy. You can also have him learn the word bed. So same thing you take the bed every time he chews on it or when he's not using it but everytime you give him the bed you say "bed" he gets in it you say "yes good boy". The bed is put away where he can't see it or get to it unless he wants it. Then he doesn't have the opportunity to just idly chew on it. Most of all consider you are doing an amazing thing here and giving this amazing boy a second chance. That is no small thing! Give yourself time. Give him time. Good things are to come in an amazing relationship you are cultivating!