I'm not sure what the plot is from what you've said

. Certainly the basic idea has been done before, so I'm sure it can be done again if you have a good plot to go with it.
The main issue I'd have is that ghosts can presumably be anywhere at any time, so you'd probably have to put deliberate barriers in the way to prevent him from solving things too easily. For example, if someone got murdered, he could obviously just ask their ghost who did it, so there wouldn't be any difficulty there.