What exactly is a man's hijab in front on non Mahram? Does he have to to cover from the knees to the belly button? And in front of mahram, you just have to cover your private parts right?

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Yes that is the bare minimum but obviously the more that is covered the better, especially depending on the context, in most cases it would be inappropriate to be topless in front of others therefore it would need to be covered too.
Same in front of mahram as well especially mother and sister.