El-Haddadi has broken into the first-team set-up at Barcelona this season and scored on his league debut for the Catalan outfit against Elche last month.
His form earned a call into the senior Spain squad for Monday's Euro 2016 qualifier against Macedonia, but as it stands, El-Haddadi is still eligible to represent Morocco through his father.
"We're playing an important European qualifier. It's the will of his parents that he plays for Spain," Del Bosque told reporters at a press conference.
"We advised him to decide freely and he is sure he wants to play for Spain. He is Spanish... he's from Galapagar."
El-Haddadi, 19, was initially part of Spain's Under-21 squad last week.