My DS8 just learned to ride a bike at the end of summer right as he turned 8. He had tried when he was younger but just stuck with the scooter because that was easier for him and didn't really give it a serious try until this summer when DS5 picked it up in a weekend. So not to be outdone by his little brother he gave it a whirl again and was able to figure it out on his own in about the span of a week.
However, around this same time is when he started doing OT for motor coordination challenges and I really do think it helped in this. Since starting OT in August he has learned to ride a bike AND tie his shoes, both of which were a challenge for him. If I remember what the OT said, I believe the core strength comes into play because it is what helps you control and balance your body. We don't think about it much and it often doesn't seem like you are using those muscles, but if they are not developed sufficiently, it can effect balancing activities like bike riding. Kids with low core strength often prefer to lie down rather than sit or they sit in a sprawling manner rather than straight up and down.
If your DS is in OT, you might ask the therapist for ideas and/or help. We ended up not needing to do it, but DS's therapist actually offered to teach him to ride a bike during therapy! She did teach him to tie his shoes which was great.
