White gums are a sign of anemia...your puppy may have worms and need treatment for that quick.
I had a puppy who had white gums and wouldnt eat or drink anything. I pulled his skin and it didnt snap right back in place so I knew he was getting dehydrated. I also knew he had worms more than likely hook worms. I gave it panacur for 5 days and also gave him nausea medicine so he wouldnt throw up. I force fed him for almost three days. After that he was a perfect healthy puppy.
We had a 3month old cocker spaniel which we were watching his fluid intake because we were trying to house train him. One afternoon he layed down and wouldn't get up or eat and that was not characteristic for him. The vet said that puppies have to drink a lot to stay hydrated. We got him some water and helped him drink it and in an hour or so he was doing a lot better. Se if you can assist your puppy with drinking some water and see if that helps. If not get him to a vet quickly. There are some puppy diseases that need to be treated right away. This can be a scary thing, but hopefully it just needs water.
White gums could mean hes either anaemic or has low blood pressure or has a reaction to something. In either case it is a very serious cause for concern and your puppy could be in grave danger! I have already lost one puppy. You must take him to the vet IMMEDIATELY!