This has been an ongoing journey with my son and it's been so much fun I'm happy to share my experiences. If he's like my son, and wants to do everything his Dad does, then you are most of the way there...and well on the road to questioning your own judgment as some of what happens will scare you.
We started with an electric Razor 4-wheeler at 3 years. (rubber tires and 5-7 MPH versus those hard tire 4-wheelers and quiet since I'm a suburbanite) The idea was my wife and I could jog while my son followed along. It turned out to be a great coordination gas, brake and the big one...pay attention to where you're going drill. We also purchased a bicycle with training wheels and it was the smallest made and the seat was all the way down. Now at 6 years old, it looks like a clown bike with my son on it.
Getting rid of the training wheels seems to be different for all kids. Some just get it and want them off, others are too intimidated. We were on vacation and my son saw another 3 year old without training wheels. My son wanted his off at that point and I was shocked when he rode off on his own. I had to run a couple miles with him that morning, as he wanted to fall to one side, I just held him up then all of a sudden it clicked and he was on his own. After watching a number of kids his age, I realize that most kids have the ability at a young age but many parents just don't try it. But it also takes some determination by the child. For my son, watching older kids was all that it took. Peer pressure can do wonders for some kids determination and ability.
Dad and son then started a little jumping for some fun on the bikes. First it was plywood with 1 brick, then 2, then 3, then we were building jumps. Just like wakeboarding, keep it fun and never push. Each kid has their own pace. It also helps to have others his age or a little older involved. The bravest in the group will help lead the way.
Then the dirt bike. I never liked the training wheels on a dirt bike and I think better skills are learned on a bicycle so we waited and my wife actually pushed the Honda 50 so at 5 1/2 he got one. I was concerned that it was too early but was I wrong. He took to it immediately but with a number of spills the first couple of outings. The falls were never a big deal to him. I kept it simple, riding wide open spaces at first. I found that the 4-wheeler experience was good for his understanding how to run the bike. Brakes, throttle, paying attention with the added complication of balance.
Its been a blast for my son and I. However, I've seen other kids try a dirt bike and some try it once, hate it, and it's all over. My son loves it. You just never know.
Here is my son at 5 1/2 years old after and his 4th time at the MX park. As a parent, it's just as fun to experience it with him as it is to ride yourself, maybe more.
I hope your experience is as good as mine has been. Good Luck!