The U21s have played two games in the World Cup, losing to Australia and defeating Italy yesterday. Scotland face England U21 on Saturday.
This will be John's second World Championship at this age level, having played a part in Scotland's sixth-place finish in last year's tournament in Mendoza, Argentina.
John said: "I'm looking forward to it greatly. Meeting up with the squad will be good fun. I'm also looking forward to the challenge of being involved in the next three games."
"It's been a tremendous experience being with the Scotland team and I've learned at lot from it."
Scotland U21 team manager John Jeffrey said: "We are pleased he's joining us as he's a vital part of our squad. We're playing in such intense heat that we need all the players we can get so we're looking forward to having him with us."