Dragon digits do some strange things sometimes. It may not be broken at all and if it is, there isn't really much you can do except let it heal by itself. Are you using any sort of reptile carpet or letting him crawl around the house carpet or furniture where he could snag a nail? He might also have snagged one on a bit of bark if you use branches. Who knows. For strong bones, make sure you follow your own mom's advice
and get plenty of calcium in him. Gutload your feeders, use low-phosphorous vitamin/calcium supplements or no-phosphorous supplements if feeding crickets, and make sure his UV lighting is optimal.