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The main problem is that Pier Giorgio would NEVER be called only "Pier" by anyone who knew him. So why do it? It's a bit disrespectful to him and to his memory to insist on calling him something that isn't right!
A lot of people don't realize that his middle name is Michelangelo. His first name is actually "Pier Giorgio." If it ever got shortened, they called him "Giorgio" -- not "Pier." If you read the book of his letters, you will see that he would often use "Giorgio." Even when he wrote in German, he signed his name "Georg" (German for "George," of course.)
Pier Giorgio also had several nicknames. When he was a little boy, he gave himself the nickname, "Dodo." As an adult, the nickname he probably used most for himself was "Robespierre" -- a name originating from the French Revolution. Although it had sinister connotations, Pier Giorgio used the name lightheartedly among his closest friends and gave them nicknames as well. When he became a Third Order Dominican, he took a religious name which he often used. It was Fra Girolamo. (On a separate note, it has occasionally been reported that Pier Giorgio discerned a religious vocation and joined the Dominican Fathers. This is not true. He was only a lay member of the Dominicans.)
Saint Pier Giorgio Michelangelo Frassati is a very long name. So, you can shorten it and use, "Saint Giorgio," "Saint Pier Giorgio," or "Saint Frassati," or "Saint Giorgio Frassati," or "Saint Pier Giorgio Frassati." But please never call him "Saint Pier," or "Saint Pier Frassati," and definitely NEVER just "Pier." Capisce?! Grazie!