Congratulations on the new addition to your family. Thank you for giving a pet in need a good home.
Hold the treats until he starts eating his regular meals. Choose a kibble and/or wet food that he seems reasonably interested in, and stick to it. Put the food dish down and leave it for 20 minutes, then pick it up until the meal time (try 3 times a day). A healthy dog will not purposely starve itself. Eventually he will eat. He is a good size, so you don't need to worry too much about a hypoglycemic episode.
However, if you keep offering food alternatives, especially treats, he will continue to hold out for the good stuff. In this case, he is training you.
You can entice him to eat by making a game out of eating. Try working on basic obedience and trick training and use the kibble as the reward. This will also make the kibble more appealing to him.