You can change it. He's old enough to go probably more than a year without eating, and as refuses, he just gets hungrier and closer to accepting what you offer him. I just got a six year old who was sold to me as a live mouse/ASF eater to eat a frozen mouse last night. He also just doesn't eat for about six months out of the year. Just wait him out and keep offering what you want him to eat.

Six mice every two weeks just sounds odd. Three every week would be a bit better, I think. More regular. If they're the appropriate weight.

Any live feeder shouldn't hurt him, if you're supervising. They are made to kill and eat small rodents. I would, however, try to switch for rats, mostly for convenience, but also for their reputed "nutritional benefits" over mice. You'll only have to feed one rat instead of 3 mice.