Going solely by top tube and head tube lengths since you don't have stack and reach numbers, the Large Ridley is roughly a 58cm Trek, with a slightly longer head tube and slightly shorter top tube (effective). There's no way I'd put him on the Medium Ridley if he is even remotely able to fit the 62cm Trek (and given that he's been riding it, I'd put him in that category). The top tube on the Large Ridley is over 30mm shorter than the 62cm Trek so unless he is riding a 50mm stem now to be comfortable, he'll be able to use a stem of reasonable length on the Ridley (90-120mm depending on handlebars).