Well it all depends how you look at it.

From the pet owner stand point I would let him leave the rest of his life and wait till he dies of old age.

From the feeder breeder stand point I personally retire my males at 9 months, 12 at the very most. Breeding take it's toll on males and I breed rats as feeders so high productivity is my goal.

Seems like you are attached to him so you could let him leave the rest of his life as a pet however it really depends on the quality of life you think he will have and you are the only one that can be judge of that.