Hang overs are physical for the most part, your brain is crying out for Water, Salts and Fat's to make up for what you have lost trying to filter out a poison, alcohol really does a number on your system especially after prolonged abuse but you can get acclimatised to them especially if your constantly doing it.
I partly suspect that's why he's drinking energy drinks with his booze beyond it disguising the taste it helps mitigate some of the effects, when I was on the sauce I used to drink those isotonic sports drinks every morning like 2 or 3 of them to be functional, the problem is he's mixing them with booze so he's not recovering and the caffine in the drinks he goes for is also causing problems for him especially with his heart rate and cognition.