To show the tooltip while hovering over disabled element, use the ShowOnDisabled attached property of the ToolTipService class! From XAML.
Syntax:

<Button ToolTipService.ShowOnDisabled="True">
</Button>

Or from C# code, you can call the static method corresponding to the attached property:
ToolTipService.SetShowOnDisabled(myButton, true);

Note: If you want to close the tooltip that is currently showing then,
Set its IsOpen property to false.