People with ADHD are usually in a state of constantly suppressing their impulsive tendencies in day to day life.
Overtime, they get much better at this to the point that their impulsivity is almost unnoticeable especially with medication.
When he drinks, he stops suppressing his tendencies and reverts to what he would be if he didn’t consciously control his impulses.
This is why ADHD is considered a disorder - the behaviour it causes is extremely irrational and sometimes dangerous.