Sorry it took so long for me to post here, many apologies!
Both of those boys drank too much. Sure, that's what boys did/do, yet they still drank too much. In the short story Jack at 19 tells Ennis they needed plenty of whiskey up there...and that speaks volumes. When Jack was bored, he drank. When Jack wasn't bored, he still drank. Jack from the beginning of the movie was a guy who just DRANK. They opened that bar at what, 7am? They were drinking as the waitress cleaned up for breakfast. The only time I ever did that i was still on a bender from the night before and I ended up in a 12 step program for alcohol and knew I needed one when i was THEIR age when they met. About 10% of the population becomes addicted to alcohol with their first drinks, and I think Jack Twist fell into that 10%. Conversely, Ennis--who in many fanfics is one hell of a drunk!--just falls into the category of a heavy hitter or a binge drinker. he could tie one on and not have it mean much. It seems as if he drank LESS as he grew older, while Jack drank more. Jack NEEDED the booze. When a Texas wife says 'he drank a LOT', you can take it as gospel:her husband drinks like a fish. Jack's general misery made him drink more....but I have a sneakin feeling that if he wasn't miserable he would have drank just as much, it was his excuse.
Being an abuser of alcohol does not mean a skid row drunk. Most --but not all--of the men I know in meetings managed to keep up the facade of normal living. Their families knew, their friends knew, 'but he could handle it' was the general comment. Except he couldn't handle it, Jack's and THEIR wives knew that it WAS a facade.
Jack couldn't handle his life, he hated every minute of it. His entire existence was a lie, and by the end he simply could not stand it, or himself. He drowned himself in a bottle before he drowned in his blood. You do not have to be perpetually drunk to BE a drunk; Jack was most certainly NOT a perpetual drunk---but a drunk he was, all the same.