Grehan initially featured as a trialist at the end of the 2021/22 campaign before making a permanent move to south London in the summer of 2022.
He was a key component of the Under-21s side last season, making 36 appearances in all competitions and helping the side to the final of the Premier League International Cup and a fourth place finish in the Premier League 2.
The centre-back has made 17 appearances in all competitions so far this season and also notched two goals.
