He didn't deliberately mispronounce his name at that point. He's just this dumb anglo guy whose exposure to ethnics is only when they are cleaning his home or mowing his lawn. He says Anthony Vy toe (long i)Corleon (like in "on" the table). In the office with Mike he then deliberately mispronounces his name by sarcastically pronouncing the e on the end of Corleone as a long e. I would have whacked him right then and there!