JOEL - The book of Ecclesiastes is grouped in the Old Testament with the Song of Solomon and Proverbs as all three of these are attributed to Solomon. But Solomon's name is never mentioned in the book, so it is not known for certain that he actually wrote the scripture.
Ecclesiastes 1:1 identifies the "Preacher" as the "son of David"(Eccl 1:1), however David had many sons besides Solomon (1 Chron. 3:1-9). Even Joseph, husband of Mary (Matt. 1: 20) and Jesus (Mark 10: 47-48) were called "Son of David", having descended from him. But the fact that Ecclesiastes was included as part of the Old Testament cannon along with Proverbs and Song of Solomon is evidence that Solomon was assumed to be the author.