While there's an antonym "composed" which is an antonym, I couldn't find a short synonym to describe someone who wears his heart on his sleeve. Can you think of one?
Such a person (and their behaviour) is unrestrained.
If you describe someone's behaviour as unrestrained, you mean that it is extreme or intense, for example because they are expressing their feelings strongly or loudly.
'Drury obviously enjoys his job ... and he is unrestrained in his enthusiasm.'
And from France 24:
Speaking to AFP on condition of anonymity, a former associate called Matovic "a phenomenon". "He is an extremely proficient communicator. He is unrestrained and sincere – sometimes too sincere," they said.
'Abandoned' and 'uninhibited' are other possibilities.
1 Expressing one's feelings or thoughts unselfconsciously and without restraint.
‘a lively and uninhibited girl’
'Abandoned' may convey too much wildness, impropriety.