Who came up with the term president?

We do not know the name of the individual who first proposed this title. Early drafts of the Constitution referred simply to the "Executive" at first leaving open even whether it should be one man or several, though the latter notion was soon dropped. The word "President" first appears in the report of the Committee of Detail, submitted to the full Convention on August 6, 1787. This committee consisted of John Rutledge (SC), Edmund Randolph (Va), James Wilson (Pa), Oliver Ellsworth (Conn) and Nathaniel Gorham (Mass). The terms Speaker, Congress, Senate, House of Representatives, and Supreme Court also appear for the first time in this report, along with expressions like "We, the People", "State of the Union", "privileges and immunities", "necessary and proper", and reference to "vacancies" in House or Senate, "disability" in the Presidency, and "extraordinary occasions" on which Congress might be assembled. So this group of relatively obscure men made a big impact, semantically at least, on the future of the United States, coining many expressions in everyday use to the present time. (All the above is taken from Ch11 of Clinton Rossiter "The Grand Convention", well worth a read if you are interested in this subject.)