Doesn't look terribly thin to me. There is a little ridging of the spine but honestly that doesn't look anything of concern yet.
I would not worry so much about switching to rats so quickly if he is rejecting them. If you feel the weight is changing too much I would just switch back to what you were feeding and try again after getting him back to what you feel is ideal weight. Keeping him a good weight and eating well while this young is far more important than getting him on rats.
For now however he still looks really healthy and his skin looks great so I am assuming he isn't dehydrated at all which is good.
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