Exercise: as much as Murphy wants! At this age, I don't push them if they seem tired, but offer several small activities throughout the day to help burn off the excess puppy energy.
Water: free choice. Leave a bowl out and let Murphy drink when he is thirsty. If he is outside for a while, water should also be available outside.
Food: it depends on the food you are feeding and the current weight of the puppy. Hill's large breed puppy is a good choice. There are feeding guidelines on the bag. For 15 pounds and under 4 months of age, Murphy should get 2 cups per day (or 1 cup morning and night). Go to this link and scroll down to "Feeding Guide" and then look at the chart, as it will change as he grows and ages.
https://www.hillspet.com/dog-food/sd-canine-puppy-large-breed-dryNO RAWHIDES! As an emergency vet, I see so many issues with rawhides, it just isn't worth it. Look for toys designed for puppies and their very sharp teeth. I like Starmark toys/treats. Treats should always be given when Murphy is supervised to avoid any disasters.
If you haven't already taken Murphy to the vet, now is a good time. He should have already received his first vaccination, and the vet will often discuss health and feeding and behaviors at this first appointment.