You need to get to the bottom of why he is not eating. That is a lot of weight loss. How old is your dog? Any vomiting or diarrhea? Dies he seem to be tender anywhere? Do you notice any lumps or swollen glands? What are you feeding? You might trying adding some cooked chicken (no skin) to his food. Buy some canned tripe, add that. Dogs usually love it and it is nourishing. If this has been going on for a while, you really shoukd get him into your Vet for a complete exam and blood work.
Please heed Margot's advice.
He needs to be checked by a vet to find out the cause of his appetite loss. Many dogs will 'soldier on' quite happily even though in a position like he is in. Dogs are known for their stoicism.
If you usually give him dried kibble....you could definitely try offering him something juicier and more wholesome and tasty! Try boiling some plain chicken or white fish (no bones!!) with some light vegetable like green beans, and try him with that? Give him the juice also.
But even so -he needs to be checked by a vet.
Losing 1/3 of his weight is not something to seek answers for on a forum. As Margot said, he needs to be checked out by a Vet. In the meantime, as Margot said, you need to try and entice him to eat to get some protein into him. Chicken is good. Scrambled eggs might be good (two a day at most IMO) along with some oatmeal for carbs.
I do find it confusing that you say he hasn't eaten for a couple days, but has lost so much weight. That doesn't make sense unless he was losing weight 'before' he stopped eating.
Just for the story: have you checked his mouth? Sometimes the first think we think about our dog's appetite is an illness- and usually it is an illness- but sometimes the explanation is more simple. My greedy dog, Charlie, stopped eating for two days and the third day we realized that his gums were swollen from an intense allergic reaction and he actually felt pain when he had to chew food. He had an antihistamine shot and then he started eating again.
However, I agree that you should see a vet.