other than getting your son to kick you in the nuts to change to voice tone you could try out this free software..
Or this on trialCode:http://www.screamingbee.com/product/MorphVOXJunior.aspx
There is a bunch of them listed hereCode:http://www.download.com/VoDi-Voice-Disguise-/3000-2169_4-10517514.html
These are untested by DK so you try them as your own risk. Be sure to always create a restore point before experimenting with software.
Good luck, let us know how you get and the product you find to be best etc..