find the carburetor and disconnect all of the fuel lines, let the diesel drain out and clean the carburetor out with "carb and choke cleaner" (available at any auto parts store). Next put some gas through the tank to drain out any remaining diesel. Then put the carb back on, fill it with premium gas, and turn it over till it starts and keep it running for a couple minutes to clean out any diesel left. Good luck!
Don't start it, it will blow up. Seriously. Im not kidding.
Seeing as to how is a 4 wheeler, you can disconect the fuel line and just dump the tank and flush it with regular gas, you might also have to remove the float from the carb and clean it with carb cleaner. also spray the jet on the carb with the carb cleaner
"your son" mhm