Seriously, Its a very annoying diseases. Not exactly a diseases, because diseases can be cured. This one has no such definitive cure that you would wanna go through. The only thing you can rely is self determination and practice. As Nikesh282 mentioned, its a self therapy. Just like changing habit, quiting smoke and these are much easier in compare to the one we are talking about.

Its hard but not impossible. It takes time but better start today. Notice those words where he have problems speaking everytime. Try to speak them appropriately first. And think of sentence using them. try it over and over again looking mirror or in presence of somebody. Sometime nervousness also leads to such habit. So be sure that u r always confident.

Remember; Everything is possible. just keep on smiling and keep on trying. This is life !

Sincerely,

Indulgent.