We were very lucky as 2 sets of friends gave us handmedown bikes for J to ride, as he was rapidly growing out of balance bike. One of them was a little too big (which is a good thing as he can grow into it) and one was just the right size for now.
The one he fits now is an Islabikes model with 16" wheels. It's a really nice little bike. He was peddling away unassisted within 30 seconds of getting on it, no stabilisers or help needed. Such a proud dad moment! Our daughter was the same when it came to getting a "proper" bike as the balance bike really does teach them that to stay up on a bike you need momentum.
Here's the clip of him riding: