Praise be to Allaah.
His prayer is valid, as was stated by al-Shaafa’i. This applies whether he was further ahead of the imaam or behind him.
From Fatawa al-Imam al-Nawawi, p. 58.
But it is not permissible to deliberately stand in a place in front of the imaam. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid