An alternative and effective option is to:
Remove the subject from your background and put him on a separate layer.
Create a new layer, CTRL/click on your "subject layer" in the layer palette and expand your selection by as many pixels as you want. (Select/Modify/Expland) You now have the "marching ants" (indicating your selection) on your screen and you have your new layer active.
Fill your selection with a "rainbow" (radial) gradient.
Move this filled layer under your subject layer.
Give it a nice "gaussian blur' et "voila".