As the PPs said, if your son wears a Birthday button (you can get it from Guest Services in the parks for sure and sometimes at other locations as well), he'll get plenty of "Happy Birthday" wishes from CMs and (sometimes) other guests!
In terms of the restaurants, it's really hit or miss what will happen. It's up to the server, the restaurant's own policies, how busy it is, etc. At a minimum, most servers wills at least say "Happy Birthday", but we've had a few that didn't even do that. Other times, we've been given a birthday cupcake, had confetti sprinkled on the table, etc. So it really varies. In general, it's best to go in with the mind set that nothing will happen. That way you are not disappointed if nothing happens.
Otherwise, you can purchase a cake and have it delivered to you table. Most TS venues have small cakes available for purchase at the podium. Otherwise, you could splurge and order a custom cake (they're pricey...as in several hundred dollars).