Nothing to worry about... it just means that he's been exposed multiple times and for whatever reason, that particular virus was able to take hold more than once. If he was constantly sick, got all different diseases over and over again, or took longer to recover than his peers, you might want to worry. But getting Fifths (or Chicken Pox) multiple times is like getting a cold multiple times. Annoying, but nothing to worry about.
(I got Fifths Disease as an adult. That truly sucked. It's way worse in adults... so just be glad you're apparently already immune)