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Rin doesn't look underweight to me, it's hard to tell from a photo, their weight does vary a lot from one ham to another but it's always a good idea to get some scales & weigh regularly to check that weight is stable.
You can tell from holding a ham if it is underweight usually, they shouldn't feel bony at all.
That food doesn't look too bad but it is low in protein, a young ham will need more supplementing than an older ham,, just a few mealworms a day & maybe the occasional piece of plain chicken or some tofu would help a lot.