I don't think he was eligible to play until the start of last season, partly because he couldn't sign a professional contract until he hit 18 and partly because he had to wait for the work permit. He made his first appearance on the bench last August in the first game of the season.
From UEFA regs:
43.11 A player may be registered on List B if he is born on or after 1 January 1994 and has been eligible to play for the club concerned for any uninterrupted period of two years since his 15th birthday by the time he is registered with UEFA. Players aged 16 may be registered on List B if they have been registered with the participating club for the previous two years without interruption.