That's right. He would be on the B list which is unlimited.
https://www.ablueheart.com/champions-league-rules-for-player-registrations
Maximum Squad size:
25 List A players (out of whom two must be goal-keepers) and unlimited List B players
Out of the 25 List A players, a maximum of 17 non-homegrown players are allowed to be in the squad. Other 8 should be homegrown.
Out of the 8 locally trained players aka Homegrown, a maximum of four can be Association-trained, other 4 should be club trained.
What is a ‘locally-trained player’?
These are split into two categories: ‘club trained’ and ‘association trained’.
What is a ‘club-trained player’?
One who, between the age of 15 (or the start of the season during which he turns 15) and 21 (or the end of the season during which he turns 21) has been registered with his current club for a period, continuous or not, of three entire seasons or 36 months.
This is irrespective of his nationality and age.
What is an ‘association-trained player’?
One who, between the age of 15 (or the start of the season during which the player turns 15) and 21 (or the end of the season during which the player turns 21), has been registered with a club or with other clubs affiliated to the same association as that of his current club for a period, continuous or not, of three entire seasons or 36 months.
This is irrespective of his nationality and age.
What is the B list of players?
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.
Each club is entitled to register an unlimited number of players on List B during the season. The list must be submitted by no later than midnight CET on the day before the match in question.