As self-conscious of his lisp as he is, it's not exactly something he's unused to people pointing out to him. This was one of the more inoffensive ways a person could point out that what he views as one of his many defects was terribly noticable.

"I have a lot of teeth and not tho much room for them," he points out in that same bland tone he tends to use for the most part, gesturing to his oversized fangs. "Therefore I have a lithp."

A pause and, for the first time since the young man joined him, he actually looks a little irritated..

"A listhp."

...close enough.