Milk all day
Seconded. Ages 14-16 I drank 0.5-1 gallon/day 2% milk. People thought I was juicing, no joke.
1/2 cup oats ground up in blender to make oat flour mixed with 1Tbs PB powder, 1 scoop protein powder and 16 oz of milk makes a good homemade gainer shake.
This^^^ Can also add in coconut oil and fresh fruit for extra cals. If you leave out the oats, you can throw the mix an an ice cream maker.
For extra cals., add in a Tbs or 2 of heavy whipping cream. Not only does it add a lot of cals, but it's also very tasty! (Learned that one from my buddy Snags years ago.)