Some countries have limits on what squad numbers that can be used in different competitions.Probably too late, number changes during a season are usually forbidden with only one case in recent memory but you'll know who and why that was.
However, he can use it in European games, I believe. I seem to remember Andy Cole having 9 for cup games but a different number in the league when he first signed for United, but that was more than 20 years ago...
In France for example, the league allows a similar number system as the one in the Premier League, La Liga, Bundesliga and so on. But for the domestic cups you are only allowed to use numbers 1-18 (or however big the matchday squad is). So it is not uncommon in France to see a player that uses number let's say 19 in the league use number 6 in the cups if that number is available.