Find a rov or someone that can tactfully explain to him that it’s likely halachically problematic to daven this loud (???? ??’ ??). Some hold of the zohar that we should enunciate the words yet not make them audible to the ear, perhaps a rov would matir him to do this from now on.
Failing that, the point of a mokoim kevuah is to aid concentration, in this case it isn’t, so you’d have to consider moving.
Otherwise try earplugs. (Seriously. One Rosh Hashana the guy behind me was davening too loudly and it being yom hadin and there is a certain minhag to do so, I discreetly put my fingers in my ears under my talis for much of shmone esrei.)