Could you give me a bit of a timeline from when you got him to when he stopped eating? Did you immediately change him to a larger enclosure? Was he on petsmart live or those "mice cube" rats they have for a small fortune? How many times did he eat for you?
Either way, 5 months isn't worrying if he is in good condition, but I wouldn't want to go a heck of a lot longer. I think him showing interest is good, and I often see this in the weeks leading up to mine that have been on a fast starting to eat again.