Psychologists view emotional maturity as "a high and appropriate level of emotional control and expression."
Emotional immaturity is the exact opposite ... so when one can't tone down the decibels when he tries to express his emotions (like when he is angry he becomes verbally assaultive ... or he resorts to violence ... when he is frustrated ... his emotional crutch is crying) when he let go of his emotions ... its -free-flowing - it is without restraint and there is no self-control.
At times when he is he is angry - his bodily reactions are over the top and professionals see those overt reactions as disproportionate to the stimuli ... either it is a small or a big reaction to a situation ... hence not appropriate. Their feelings of joy, happiness, anger, and sorrow are either stronger or weaker than average. For instance, the stimulus is the death of his favorite pet ... or an ex-girlfriend snubbing him at a party ... if he goes ballistics and if the resultant behavior is undue and bordering on the extreme ... like breaking items of furniture ... such reactions are exorbitant and unnecessary.
Your observation of his emotional responses is the hallmark sign you are dealing with an emotionally immature individual. You will observe his feelings can easily be jacked up ... or it can go through the roof ... in the same way a child would react. They have no shame in throwing unpleasant and disruptive behavior and outbursts ... his crying spells are on cue ... and when angry he can show childish and juvenile antics when things do not go the way they want them to.
Emotionally immature people are often seen as lacking emotional sensitivity and behaving in an absorbed and occupied with oneself ... you will realize that any form of communication is next to impossible. It is something one has to learn in order for you to deal with them effectively.
Experts always say that emotional immaturity is not a diagnosis per se as a mental health disorder ... but this behavior is a part and parcel of a narcissistic personality disorder or borderline personality disorder.