My puppy won't eat.

What is the switch that you have made? Often times if you go from a cheap brand to a high quality brand they'll protest a bit in the beginning as you're essentially taking a child from French Fries to Broccoli.

If you've merely switched him over from a puppy food of one brand to an adult food of one brand, then something else may be going on and I'd check to see if there are differences in ingredients that could be bothering him.

When you were switching him over and mixing the old with the new, did you gradually phase out the old so slowly there was more new then old in the mix or did you do pretty much 50/50 between the two until the old was gone and then did 100% new? That could make a difference as well.

Is it dry food? If it is you can always mix a bit of water during the first couple of days and then try to progressively add less water until he eats it. I usually do that to my dog and it seems to do the trick.