Well, just to add another dimension to this whole debate:
At my 5y/o's recent checkup (when he turned 5), the pediatrician asked SPECIFICALLY:
"Can he pedal a tricycle or bicycle?"
It's considered a developmental milestone, at least by 5 years old.
SO, just from that perspective, even if your child doesn't have the INTEREST to ride a bike, I think, developmentally, by 5 years old, he/she SHOULD BE ABLE to pedal successfully. Same as being able to throw/catch a ball. Kick a ball. Climb up/down stairs, etc. These are basic gross motor skills. By a certain age, these milestones *should* be reached. If a child fails to reach them by a specific age, it *could* signal a gross motor delay, which really should be investigated further by an OT evaluation.
So, not to go too far into the deep end, but keep that in mind. Could be an issue with the large muscles, coordination, core muscles, etc. It's not something you should just push aside as "nothing", because if that is the case, it will affect more than just the ability/interest in riding a bike, and it will come into play EVENTUALLY when he goes off to school and is expected to participate in activities requiring strength and/or coordination that he may not have, and you don't want to put him in that position.