There's no real way to do that that I can think of. There is also no way I know of for signifying a whistle using only English letters, so the only thing you could really use here is to say that the character has the lisp afterwards. For example, if the character says,
"Green potatoes are more like teddy bears,"
you mights afterwards say something to signify his lisp, like
A severe lisp had turned all of his S's into whistles,
or, if we know about the lisp already, something like
The whistles his lisp somehow produced turned his very serious statement into humor.
If you don't want to do that (it does seem sort of tedious to write and read) you might consider changing the lisp to something easier to write, like if S's turned into Th's, or Sh's, or something; or, as this can be hard to read, just remove the lisp altogether.