Noel,
You will need two forms. You are basically selling the airplane to yourself and your son. You will need to fill out an aircraft bill of sale, FAA Form 8050-2, as the seller. Then you'll have to fill out an application for aircraft registration, FAA Form 8050-1, showing yourself and your son as the applicants. No money needs to change hands, and no price needs to be listed on the aircraft bill of sale. (Most people just put "$1 + OVC" in the price slot on the bill of sale no matter who is selling to whom.)
Both forms can be obtained via the FAA website. Just do a search for the form numbers.