Make sure you puppy has access to fresh clean water at all times. Water should NEVER be withheld to aid in potty training. The only time my dogs or puppies do not have access to water is when they are crated, and for very young puppies this should be for short amounts of time only.
When choosing a food for your puppy, make sure to feed a quality kibble with no by-products or added chemicals. It is also good to go ahead and start your puppy on adult food, not puppy food (your vet may argue with this, but they aren’t giant breed specialists, remember these aren’t large breed dogs, they are giant breed dogs). Most puppy foods have protein levels that are too high for giant breeds that grow so rapidly. You want your protein level to be right around 25% and absolutely no higher than 28%. It is also important to keep your calcium/phosphorus ratio right at 1:1. Doing these things is a huge step in preventing conditions like HOD (Hypertrophic Osteodystrophy) or Panosteitis. These are both painful conditions and can be crippling for life if not treated with a balanced diet.
Remember, the size of your Mastiff is genetic, not nutritional. You cannot hurt your puppy by keeping him lean and fit, but overfeeding a Mastiff puppy can ruin them orthopedically for life, causing a lot of pain and numerous vet bills over the life of your Mastiff.