Huh? According to the official Patiots website he's a "real" FB:
James Develin Interview - 9/3/2012
The answer to your question is because he's been here off and on for a couple training camps, knows the complex system, and is currently been getting increased reps that have come with the first team.
I'm not saying that a FB will definitely make the squad, but if a FB does make the squad it's going to be James Devlin--not a guy they are going to bring in this late that will need to
attempt to learn the system from day one.