He must be high or just plain stupid
I think "delusional" might more adequately describe it. Most of the Star Wars/Sci-Fi conventions I'm aware of aren't sponsored by Fox, LucasFilm, Ltd or anybody who might have contractual authority over whether or not Prowse attends or not, and so Lucas would really have no say in what Prowse does in his "free time".
Prowse should seriously tell Lucas to pound sand.