In general Highcharts doesn't support tooltip on labels.
However you have two solutions:
- use
Custom-events
plugin, to add event on labels: https://www.highcharts.com/products/plugin-registry/single/15/Custom-Events (demo has click, mouse over should work the same to display custom tooltip) - use some plugin (Tooltipsy etc.), like this: http://jsfiddle.net/gW4p6/196/