First of all, Harvard offers Restrictive Early Action, not Early Decision. This may sound hairsplitting, but there’s an important difference because Early DECISION requires a binding commitment while Early ACTION allows admitted applications to wait until May 1 before enrolling.
But you probably already know this, even if your terminology is incorrect. I realize that this isn’t what you’re concerned about. So, to answer your question …
Your son can submit his REGULAR DECISION applications to anycollege at any time. He doesn’t need to wait for a verdict from Harvard. However, if Harvard is his first choice and he’s sure to go if admitted, you might as well save some dough on application fees and hold off on those submissions until you hear from Harvard. If, however, he plans to send in the Regular Decision applications anyway (perhaps to expand his options or to compare financial aid or merit scholarship awards), then there’s no reason to wait. Of course, he will definitely have to send his UC applications before hearing from Harvard because of the November deadline (and, again, there’s no prohibition against this).
Harvard’s Restrictive Early Action rules also allow students to apply Early Action to PUBLIC universities … just not to private ones. Students can apply via non-binding Early programs to foreign universities as well.
So please assure your son that finishing and submitting his Regular Decision applications will have no impact whatsoever on his Harvard outcome. As noted above, I’m not a big fan of sending these out before the Harvard news rolls in (except when the deadlines require it, such as with the UC schools, or if your son is certain he wants to apply to these places regardless of what Harvard says). However, I do endorse getting the regular applications all ready to go by mid-December. This way, your son hasn’t wasted money on applications to colleges that he’s certain he wouldn’t pick over Harvard. But, on the other hand, if he doesn’t get good news in the Early Action round, it’s far less depressing to have to simply SUBMIT the remaining applications than it is to actually have to DO them!