Tell him that you have an offer from another company on the table and that you simply can't wait around and pass up other opportunities while they keep you on ice with little communication. Essentially, give him an ultimatum of needing an offer letter by the end of the week. If they are for real, they will react.
Sorry bro, sounds sketchy. Especially if the CEO is giving you the run around. Doesn't sound like a company you should work for if they can't even recruit a new hire properly... imagine how they'd treat you if you worked there.