Not good for them to have too much human food, no. Especially anything pre-prepared from a packet or can, like soups, since these tend to be high in salt and sugar, both bad for dogs. Onions, garlic, tomatoes and several other ingredients we use a lot in our own cooking are also toxic to dogs, so please, please look up what foods are unsafe for dogs and avoid at all costs!
First thing - has he had a vet check since you adopted him? A trip to the vet to ask to have his teeth checked especially, and a general health check should be the first port of call. If his teeth or mouth hurt when chomping hard food, that could be the underlying problem. The vet can also give you advice about his diet.
What kibble food are you trying to give him? Will he eat any dried dog food products, like treats? Have you tried giving him some of the Caesers dog food mixed with some of the kibble?
As for training him, don't indulge him with the human food, and don't feed him from your own plate etc. You can spoil them and basically train them to resist their own food because they know you will give in and give them the exciting human food in the end. You can't give in to that.
Assuming all goes well at the vets, and it's not a problem with his mouth, then try a mix of the wet food and the kibble. Put the food down at his mealtime (and have set mealtimes, twice a day) and give him 20 minutes to eat it. Once 20 minutes are up, take away the bowl even if he hasn't touched it, and nothing else besides training treats if you're training, until the next mealtime. Keep it up, and he will learn to eat in that time slot, and he'll be hungry enough to eat because you haven't been giving him lots of human food treats in between.
Exercise - If you take him for a good long walk an hour or so before breakfast or dinner, play with him, really get him to burn some energy, he'll be hungry when his meal is given to him.