How do you supervise your former supervisor?

Wondering the best way to handle a situation where you now have to supervise a guy who was your supervisor. After having my position elavated, i now have to make most of the decisions on operations of the IT department. This includes having to give directives to this former supervisor. The guy seems not to have accepted that this is the new order of things and still thinks of himself as the ‘decision maker’. He makes serious blunders whe it comes to decisions, and reasoning with him on issues always complicates things more. Anybody out there who’s faced such a situation. Whats the best way to deal with this amicably?