|The reason your employee is asking for more extensions is that you have been extending them. You have essentially been rewarding the behaviors you wish to eliminate.|
Your desire to avoid being a “tough guy” may be masking a need to be liked. This type of need prevents many managers from reaching their full potential and effectiveness. And, by the way, employees do not necessarily like managers who agree to every request.
This does not mean you should act like a dictator, but it does mean that you should be more direct and assertive in dealing with this employee. Sit down with him, review the status of all of his projects, reset some priorities and deadlines if necessary, and finalize a due date for the project in question. Unless there are extremely extenuating circumstances, there should be no more extensions on this project, and you should think very carefully about further extensions.
You should also increase your follow-up with this employee, and if he still continues to fall behind, there may be questions as to whether he should continue at all.