Oh damn... He's quite stunted and sickly. His slime coat looks a bit messed up towards the tail; may be an infection of some sort. Perhaps the red spots may be some sort of lesions that haven't healed properly or another indication of an infection. I would recommend that you separate the fish from the others, place it in a quarantine vessel of some sort (I often use large plastic bins for a temporary quarantine tank) and first attempt to feed him a good amount of some sort of frozen/live food; perhaps earthworms, brine shrimp, f/t silversides, etc. It's a bit concerning that your bichir allowed you to flip him upside down underwater, that he's that small and skinny, and his physical condition. If you cannot quarantine him, I think the first priority should be feeding him an appropriate diet; for a fish that eats well does not often run into too many health problems long-term.
Not in any way a fish vet, though. I'm not completely too sure what the physical health complications are; all I know is that you really need to get his weight up and probably quarantine and treat him with the appropriate meds. Could be parasites, which would explain the skinniness, but could also, in turn, be neglect, the wounds.