lockjaw

My arowana can't close is lower jaw but it is eating fin, What could be problem to him/ no bacterial or fungus issue any advises

hi i was feeding my endli frozen fish earlier today and i noticed he was approaching a piece of fish but his mouth was closed. a while after he wasn't noticeably full. and i noticed my othe polys noticeably pump water over their gills even when idling causing the mouth to open and close...