I know Dortmund is a good team but I am not sure if this is the right move for him because Sunderland just got promoted to the premier league and making appearance in the premier league will make him exposed and easily catch the eyes of bigger teams instead of moving to Germany.
Sunderland isn't automatically qualified to Premier League, they still need to win playoff tournament against Sheffield United, Coventry City and Bristol City. Moving to Dortmund is better, his brother played in Championship team and trained in Dortmund which make him to be like that.
This might be the reason why he move to Dortmund, young brother then to follow his older brother journey.
Now which European club will bring him in because I haven't found the latest news and hopefully Napoli and Osimhen find the best way for the player to leave as soon as possible.
Liverpool, Arsenal, Chelsea, Manchester United, Tottenham and Aston Villa are rich teams who currently lack of striker, but surprisingly haven't make a move to sign Osimhen.