If he is 5 weeks old, and has teefers, go ahead and buy him a high quality puppy food (preferably NOT from a grocery store; go to a feed store, or a specialty pet store) or an all life stages food like Canidae... and add warm water and make it mushy for him. Adult dog food is fine, for a couple days. You can also add a little high quality canned dog/puppy food. Just make sure he stays hydrated!

About the crying, he probably misses his mom & siblings. If you have any hot water bottles, you can wrap towels around that, and put it in the crate. I have also heard that putting a clock (that ticks) in there can soothe puppies.

It's really nice that you took this puppy in! I would keep him away from the other dogs, before he visits a Vet, just to be safe.