I have 2 dogs that are rescues. Here are a few things to think about. (1) He is a puppy (2) even thought that is the food that they gave him, it might not be the only kind. Some shelters give what ever is donated, so it can change. Plus they don't alway have the best quality food. (3) how long was he at the shelter- this has 2 issues, 1st he could have had other food at his past house or have been on the street and had to eat scraps. The other is going from one enviroment to another (especially a shelter), then to your house is scary. (4) Fear, he might not trust you enough yet to be sure that it ok to eat the food.
My 1st rescue was a bit of a problem and we had to try a few things, we finally went to wet food b/c she would eat the dry. It's taken over a yr to get her to eat both wet and dry.
Our 2nd rescue was found out on the streets of Philly. She really wasn't sure what food was and was so skinny. We had to be careful not to feed her too much, too fast or it could really upset her system. But we didn't have to worry too much about that b/c she would eat any dog food. We finally decided to try a little boiled chicken. Taht she ate (since she probably was dumpster diving, that was more what she was used too). I would put in less chickan and more dog food. I'd even hand feed her the dog foor (it was wet chuncks cut small- gross but she was so skinny and a skinny chi at that). It took several weeks to get her to eat on her own and I finally could put just dog food. She she gets scared at times. We have to put the puppy away, and she's just getting to the point that she is ok w/ the kitchen, and I still have to be in the kitchen but not look at her to eat but she does pretty good now. Yeah I know she is a little odd, but she is getting better everyday. The most important part is patience and take it slow.
Good luck!!!