What causes low self-esteem in a guy?

I'm very concerned about my younger brother. Both of us were raised with the same loving family but he appears to have little to no self-esteem. He lets the same people that used him get back in his life again. All they need is to say sorry and that's all. He kept taking a cheating girlfriend back until the girl herself dumped him in the end and even claimed to only be dating him out of pity.

He just turned 19 recently and the path he's heading towards seems self-defeating. He has group of friends (very few though) but he literally has little to no backbone. What causes this in a guy? My parents are worried too. Any way we can help him get stronger (I don't mean physically but mentally)?
What causes low self-esteem in a guy?
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