What is his weight?
What I would recommend is to feed adult mice once a week for 3 to 5 meals give him time to adjust to the size. Once used to the size offer a rat that is the same size as the mouse offered and move up in size accordingly (see our feeding chart in the husbandry section)
Hoppers are what hatchling out of the eggs are offered so yes this animal has been severely underfed, and the priority IMO is to get the food size right rather than attempting to switch which can take weeks or even months and means no food during that time until the animal switches.
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