Going from 100g to 180g in a year is very slow growth.

At 180g he should be able to take a rat pup easily. Definitely an adult mouse at the least. Feeding him the size of the fattest part of him was correct.

You can also up his feeding frequency. When they're young, you can feed them every 4 or 5 days without issue.

Short story, I'd start feeding him a rat pup every 5 days and increase the food size as needed. He should catch up in no time once he's on a better feeding regimen.