My son's 14, 6ft tall, hit puberty at 10 or 11, and is starting to grow facial hair. pretty sure he's stopped growing but could he still be taller?

A bit: It sounds like he has reached his adult height. An xray of his long bones would identify if his growth plate is still active or if he has stopped growing.

Depends on variables: Skeletal maturity, not facial hair, determines the duration of linear growth. If the bone maturity is ahead of a persons age, they finish growing sooner. An x-ray of the wrist and hand can be analysed for open growth centers. If they are closed growth is finished.Most males are done at 17-18, a few with delayed bone maturity continue to grow slightly.