Hi, Kendra. I'm happy to hear about Mack. I honestly wouldn't know what to tell you. Under normal circumstances, Mack's vaccine should last for five years or more, but considering he had parvovirus and his immunity is low, I don't know with any certainty what to do.
Katie is five years old, and she was due for her booster in May 2013. I had her titered then and also this May. Titers aren't fully reliable, but considering the duration of immunity of these shots and the fact that she is healthy, I feel more secure with the titers than not testing at all.
I've attached a link about titers.
Dr. Jean Dodds' Pet Health Resource Blog | Avoid unnecessary vaccines with titer tests (Part III)