It is wrong for anyone to offer prayers loudly unless he is an imam leading prayers in a congregation when and where prescribed or for the purpose of teaching.
The Prophet (peace be upon him) said, "Pray as you saw me praying" (Reported by Bukhari).
Therefore, the companions prayed as they saw him praying; and they transmitted for us as diligently as possible the precise manner and method of his prayer. We never find in any of their reports any precedent for such a practice: a person praying loudly in the mosque unless he is leading prayer or instructing others to pray.
Furthermore, the Prophet (peace be upon him) also warned us not to disturb other worshippers even by reading Qur’an aloud.
Therefore, I can only see one reason for his behavior: Perhaps he does not know what he is doing is wrong. So, I would advise you to go and tell the Imam to correct him confidentially. If the imam is not available, you may do so gently and wisely without exposing him.