Mouthbrooders will circulate water in their buccal cavity (throat area) I am not talking about breathing per se although it could be mistaken for laboured breathing. Mouthbrooding behaviour is usually distinctive too. Fish stop eating in early stages and are often secretive and reclusive. Youve stated that you already know hes holding though so I will stop my explanations.
if you really want too know, try feeding him... if he don't eat, he's holding... usually mouth brooder's don't eat when they are holding for a few weeks...
andy