He should have water available at all times. Make sure to ensure it is clean.
The amount of food given should be as directed on the bag, or can of puppy food that you purchase. Below is a good article that gives some good advice about buying pet food:
https://www.petcoach.co/article/what-to-look-for-when-choosing-your-pet-s-food/Treats can really be anything (chicken is fine). The safest bet is the manufactured treats at the pet store. However, if your puppy will eat carrots, apple, or other veggies/fruits, then these are a healthy alternative. Overfeeding treats is a common reason for dogs to get overweight.
There are some foods that you need to avoid feeding your dog. Below is a link to an article that will tell which foods to avoid:
https://www.petcoach.co/article/10-common-foods-that-are-toxic-to-dogs-and-cats/I hope that helps and all is well with Axel.
Just as a reminder - he should have his first vaccines, deworm and possibly other puppy treatments at eight weeks of age. It's best to schedule an appointment with your vet if you haven't already.