Whatever he can see better with.
The vertex distance usually does not cause any distortion. On Rx's where it might, the actual vertex distance is measured and the Rx is adjusted for it. The base value labs use for vertex when not specified in fitting is 12mm. The Rx usually has to be above + or - 8 diopters before the vertex needs to be moved from 12mm.Absolutely stay with the contacts and use plano lenses in shooting glass frames.
Consider: the contact corrective prescription is right up against his eye....with prescription lenses, there is a small but actual distance between his eye and the corrective lens which likely distorts the image ever so slightly. I used contacts for years until I had cataract implants. The contacts worked fantastically well! Plano tinted lenses are less expensive, too.