If you have a character with a speech impediment, it most likely will be distracting — and quite possibly condescending — to try to mimic the impediment in all of his dialogue.
Instead, include mention of the character’s speech impediment when you introduce him, and then write the dialogue as normal, doing your best to capture the character’s voice (read: syntax) without trying too hard to mimic speech patterns. The director and actor will take care of that.
In a 2007 [blog post](http://johnaugust.com/2007/short-answer-sprint), John August breaks it down simply:
>> * Use the speaker’s words
>> * Use the speaker’s grammatical structure
>> * Don’t try to duplicate the exact speech pattern on paper
>> If you have more than two apostrophes in a line of dialogue, you’re probably overdoing it.
If a character who does not normally speak with an impediment develops a temporary one for whatever reason (a cute girl makes him stutter, he has a mouth full of M+Ms, a punch knocks out a tooth and he has a temporary lisp), you can do your best to include it in the dialogue if it’s appropriate, or you can include it in a parenthetical or in the action lines.