I see you have plastic plants in there, are they missing any leaves? I have had a young flowerhorn rip a leaf off a plastic plant and get it lodged in her throat. Looked similar to how your RD looks, throat flared at all times. I could see the leaf in her throat though.
I ended up doing minor surgery with a pair of tweezers, and pulling the leaf out of her throat through her mouth. She was fine afterwards. If you can see anything in there, this is what I would recommend. If you cant see anything, try pulling him from the tank and looking in his mouth with a small flashlight.