I apologize, I don't remember how old your son is, but I feel like the first part of my answer probably won't apply to you.
If exposed to a bike/trike/whatever with pedals, it seems like most kids will master pedaling between the ages of 2 1/2 and 3. I would definitely say this was the case for my son. He got his first bike at 3 1/2 (having all ready mastered pedaling on a trike) and was riding it with training wheels easily.
I will also tell you that my best friend's daughter had not yet mastered pedaling (she was 4 1/2) at that same time. I don't know why. She's 6 now and she can pedal fine.
I'm guessing, though, that since your question is listed under the "preschooler" tag that he's older than a toddler.
On the plus side, since your son had a balance bike, he's presumably mastered balance and steering pretty well, he's just unsure of how, exactly, to propel himself forward. I can't find any resources online that specifically addresses how to teach your child to pedal. Most resources focus more on teaching your child how to balance the bike, assuming that your child has all ready mastered pedaling before you put them on a regular bicycle. So my suggestions are going to be things I can think of and you may or may not have tried some (or all) of them all ready:
- Demonstrate If you own a bike (and I'm going to assume that you do), then take it with you when you go to work on learning to ride. He may just need someone to show him how this whole pedaling thing works.
- Explain If your child is somewhat mechanical (and even if he/she isn't), explaining to your child how a pedal works might help. He's older, so if you explain to him that you're simply pushing the pedal and it moves the wheel, that might help him grasp the concept a little better. The key is he has to understand that he has to do this to both pedals to keep the bike moving.
- Determine Is it that he finds pedaling difficult to grasp or is he afraid of falling? Sitting on a bicycle and sitting on a balance bike are a different feeling, so even though he's presumably mastered balance with the balance bike, sitting higher up on the bicycle might be shaking his confidence a little. If that's the case, I'd throw the training wheels on there to make him feel more comfortable. Once he's mastered pedaling, removing the training wheels shouldn't be too much of a problem since he all ready understands balance and steering.
- Practice You can practice "pedaling" without sitting on a bicycle. When I was a kid, my grandmother and I would lie on the floor or on the couch with our feet facing each other. Then I would put my feet on her feet and we would "pedal" in the air. I thought it was awesome fun! It will help teach his muscles the motion that he needs to use to push a bike pedal, and you can adjust the speed starting off slower and gradually increasing. It also gives you the added benefit of being able to mirror his movements and you can explain while you're going along (ie. "bend your knee, now straighten it out" etc.) until he gets it. It's a little more low-pressure than sitting on a bike.
- Attend If there are any biking clubs in your area or bike safety days in your area, go and take his bike. Seeing other kids around his age biking might be the push he needs to tackle pedaling. Or schedule a little play-date with another kid who is biking. The other child doesn't have to be biking on two wheels, just pedaling since that's the skill you're trying to encourage. Sometimes other kids are the best teachers for things like this.
- If all else fails, buying a Kettcar doesn't look like a bad alternative.
Like a lot of things, I'm willing to bet that once it clicks, he'll be off like a light.