Just tie a rope to the tube under the rear fender and jog behind him for a while. Tons of kids 4 and younger riding them without the remote kill switch contraption.
The keys are to use the throttle stop. Use the power limiting exhaust washer that comes stock. The bike will barely crawl with both installed and adjusted. Your kid could literally go faster on one of those battery toy bikes from wally world.
We used these methods for my 3 year old year old. We used training wheels for about 1 month and then ditched it. I needed the excersise anyways, besides...just because you kill the motor does not mean the bike stops! The rope method you can slow the bike while coaxing the rider to use the brakes.
Put the bike on the center stand and practice revving and braking to get used to it BEFORE letting him ride. Practice for a few minutes every time you go out for a ride. Little junior will pick it up right away.
These methods took my 3 year old from the pw to winning some national moto's, several national podiums and a Factory ride with Polini. He's 8 now. don't under estimate your little guy. They will surprise the heck out of you.