Q: My client had a stroke when he was a baby, and he has both slurred speech and a lateral lisp. Do you think he can learn to develop a central groove for the sibilants?

Only time will tell. In the meantime, focus your therapy less on individual phonemes and more on improving intelligibility by helping him learn to speak up, speak out, and over-exaggerate. Exaggeration is the method recommended most often for clients with dysarthria.