Wikipedia (cuidado!) covers ceceo in Spanish (link) and English (link). And a linguist describes the phenomenon in Spanish (link) at an Austrian academic site. The reason I personally doubt it, as pointed out by twosleepy (welcome) in this thread, is that if the putative (unnamed) king actually lisped, he wouldn't have just lisped sibilants represented by c and z in Spanish orthography, but also those in s. (Sarà vero ma non ci credo.) Also, note that not all continental Spanish dialects exhibit ceceo, not even in Andalusia.

Ceci n'est pas un seing.