You don’t learn cycling with training wheels. The most difficult part of riding a bike is keeping balance and making a turn, not pedaling or braking. When you have appropriate speed, an experienced cyclist performs a steering manoeuvre called countersteering (although most of them are not aware of that): In order to turn left, you push the left bar handle (yes push, not pull). That way you "fall" to the left and at that moment physics kick in and you make a nice left turn.
If you have training wheels, you are learning the exact opposite: if you want to turn left, swing your steer to the left, if you want to go to the right, swing it to the right as if you are in a car. So you'll have to re-learn cycling again once the training wheels are dismounted.
Learning the hardest part (keeping your balance and steering) can best be done on a low bike with the pedals dismounted, using your feet to make speed. If he is experienced in that, just remount the paddles back and off you go!
Source: Living in the Netherlands, father of four kids that could all ride a bike at the age of three (without any use of training wheels) and being an active motor bike rider.