Going out with a recent Chozer Biteshuva, the boy wants to know the exact Mekor of this habit or Halaha. Is it a humra? What are the real limits? holding hands, kiss on the forhead, etc. If possible, I would appreciate if the rabbi gives me all the mekorot so that I myself can also study them. I can read and understand jewish sources. Thanking you in advance.
See Rambam Hilchot Issurei Biah ch.21, and Shulchan Aruch Even HaEzer article 21. The examples mentioned in your question are both forbidden by halacha. One should not examine the halacha to determine what is more serious and what is less so, so as to keep only that which seems more serious (I assume that is not why you are asking the question). The bottom line is that all physical contact that may in some way be a manifestation of affection is forbidden between a man and a woman who are not married and are not, for example, a mother and her son.