Have you tried rats instead of mice? At 600 grams, you could feed him a crawler (leave it in for a while at night). If a crawler is too big, then go with a fuzzy or even a pinky.

If you have had some RI's in your collection (I didn't see the other thread regarding the cause), it could mean that something is off in your husbandry. If that is off, then it isn't unusual for them to go off feed. If you can get the husbandry spot on, it would help with feeding issues.

Good luck.