First you need to ditch the screen cage. Mangroves need a humid environment and you won't be able to keep any humidity in that type of cage. They are also nocturnal so he won't be moving during the day. He appears to be of a decent size/weight so he's probably in pretty good shape. If he hasn't already been I would probably worm him. Plenty of people on here keep mangs so they will probably chime in. I've delt with plenty in the past.
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John Light