He's certainly looking good now, and we can throw the idea that he was underfed out the window, lol. Those folds you're seeing are perfectly natural. In fact, you'll notice some vertical 'bend lines' on the sides of most Boa constrictors - they love to pack themselves into tight spaces and that causes 'fold' lines on their sides. Again, that's perfectly normal, so overall I'd say you little guy is very healthy and you can relax and just enjoy him
I often call those vertical fold type lines 'pack marks' because they get them from packing into tight spots. I'm not sure if there is a proper name for them, but if there isn't, there ya go, lol.
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