He is sad, he bursts into tears and when happy his heart knows no bounds. He is that transparent person when it comes to emotions. An open book!! I am looking for a word for it, if it exists.
I like mercurial for this purpose. Link.
subject to sudden or unpredictable changes of mood or mind: his mercurial temperament
(of a person) liable to display rapid changes of emotion. OED
Labile would be the appropriate psychiatric term. Otherwise, are we talking somebody diagnosed as bipolar (mood swings on the order of months to years)? In that case you can use the colloquial "manic-depressive." Using mental health "diagnoses" outside of actual psychology as part of an expression is offensive to the mentally ill and their providers. As such, I downgraded your question. Disinhibited is also appropriate (without the hyphen).
Someone who suddenly bursts into tears when they are only sad, or appears to be greatly animated when they feel happiness or excited about anything can be described as
acting manic: showing wild and apparently deranged excitement and energy.
A more sensitive expression without the negative connotations is effusive
Showing or expressing gratitude, pleasure, or approval in an unrestrained or heartfelt manner ‘an effusive welcome’
I guess it's impossible to find out a better word than : bipolar http://www.nimh.nih.gov/health/topics/bipolar-disorder/index.shtml