I agree, he may be afraid of failure.
Allow him to let you know when he is ready to try.
Also let him know. most people really have to practice for a while, but that is ok. It is just part of the learning process.
Maybe suggest you all go and practice in an empty parking lot (away from prying neighborhood eyes) so he can have lots of room without obstacles.
Our daughter was the same way. She can be a perfectionist and likes to be able to "do it right the first time", but she is like me and very Klutzy and uncoordinated so she was not too excited about failing.
We also at that time did not have sidewalks only the street so that freaked her out.
We used to take her to the Cemetery to practice.. It was not till she was about 13 that she was able to ride, but was not very confident. Up at college, some of her friends have been working with her every once in a while , hee, hee, I can hear the squealing down here. I hope she learns by the time she graduates. Otherwise we are thinking we will have to purchase one of those old lady 3 wheelers!