My boyfriend learned to ride a bike last summer, at age 25. He found some person on reddit who lived in the area and had an extra bike, and they went out to a big empty parking lot for a few hours. The person helped him balance while he got used to pedaling, and gradually he was able to stay on. After that, we just started going on practice rides around the area, and a few weeks later he was fine.
I would recommend picking up some bike gloves - he had quite a few wipeouts while learning.