The slit on his belly is indeed his belly button.
It does look like your snake has really minor scale rot on a few scales. (Hope I'm looking at the picture right, it's kind of small). The rest looks just like natural markings.
It's so minor that it should go away with the next shed. I wouldn't worry too much about it.
If you really want to do something, you can dab some diluted Betadine on it. Betadine is an antiseptic you can get from your local drug store.
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