In the button's MouseMove event handler, make a label containing the help visible. Also make a rounded shape control visible to outline the help.

Under the button and extending outside it some distance, put a imgNoTip control. In its MouseMove event handler, hide the balloon.

I would make one change. If you move the mouse quickly enough off of a control that displays a balloon, the imgNoTip control around it may not get a MouseMove event. In that case, the balloon stays visible even after the mouse moves away.

A more reliable method is to start a Timer for some short period (say, 250 milliseconds) when you make the balloon visible. When the Timer fires, it uses API functions to get the mouse's position and see if it is still over the button. If not, it hides the balloon and disables the timer.