If its 131g it is probably pretty young unless it is emaciated. Most likely it is a 2011. You mention that the skin is saggy. Can you post pics? Could be lots of reasons.
As has been said, feed it a meal that is as big around as the fattest part of its body or roughly 10-15% of its body weight, so 15-20g meal would be perfect. Every 5-7 days is fine. On that note, I'd recommend doing away with the mice. Offer him an appropriately sized rat. Rats are more nutritious than mice and making the switch from mice to rats later in life can be much harder. Its a real pain having to feed an adult ball 5+ mice in a sitting. I've been there. After the switch you'll notice a big difference.