I'm so sorry to hear about you little guy.
it's so sad and my heart goes out to you.
The things that i've found out while on this site about the symptoms that your little guy is experiencing are as follows:
-Irregular swimming is often a serious sign of respiratory infection. Fluid build-up in the lungs impairs normal swimming ability. Irregular walking may occur due to the same reasons. Check for other symptoms, take basic measures and prepare for veterinary consultation. Make sure the water temperatures are correct and that your turtle is not exposed to cold drafts.
-These types of irregularities may also be due to a neurological condition or an ear abscess. This is a serious symptom that needs immediate attention. It can easily lead to drowning and other injuries.
( have you taken him to the vet an ruled out an ear abscess? You might not be able to tell just by looking at his head because it might not show.)
-Respiratory infections (RI) are common and deadly illnesses that affect many captive turtles. It is a contagious illness that can spread to other turtles and often requires medical attention. Improper basking and water temperatures as well as exposure to drafts and breezes greatly increase the chances of contracting RI. There are varying degrees of RI, including pneumonia, and the symptoms may also differ.
-The most obvious symptom that can signal RI is if a RES is listing. Listing refers to swimming in an irregular manner, such as swimming in circles, unevenly or lopsided and even swimming upside down. Listing is caused by fluid in the lungs; more specifically, if fluid exists in one lung or there is more fluid in one lung than the other. Your RES requires immediate veterinary assistance if they exhibit this symptom. They may have RI but not this symptom. The sooner that they are examined, the better the chance they can recover. They would likely be administered Baytril or another anti-infective. A vitamin injection, antibiotics and an x-ray may also be used or suggested.
-There are other possible symptoms that can signal RI, especially if they occur frequently. These may include breathing difficulties, coughing, sneezing, open mouth, yawning, wheezing, lethargy, loss of appetite, mucus from the nose (runny) or mouth and bubbling at the nose and mouth. If your RES exhibits any of these symptoms, then raising temperatures, quarantine and a water change would be the immediate actions to take. Ambient air temperature and water temperature should be raised 10 degrees F. Eliminate any drafts and cold air around the tank. Pneumonia may develop which in many cases is fatal. If your RES continues to exhibit a symptom or develops more, then consult with a qualified vet immediately.
(have you also ruled out RI with your vet? Symptoms may not show, so an X-ray to rule out RI and fluid in the lungs may be necessary.)
- since your turtle has had a lot of trauma because of the tail being bit off by another turtle, it might also have an infection from that same thing. Infections aren't always seen where the scratch or cute happened, and in his case, even if the tail area that was bit off looks to be healing, it might be an inner infection, one that you cannot detect just by looking at the area. With his tail being bit off it is possible that he might have Septicemic Cutaneous Ulcerative Disease. I know that is more associated with the shell, but it is possible. Have you had his blood tested to check and make sure everything is normal? All of his symptoms could be several different things combined... unfortunately, i think you might have to do a full work-up at the vet's in order to rule out everything. I know this will probably be costly, and i'm not sure what you have already had done at the vets, but if you really want this little guy to survive, i think a full work up at the vets would be your best bet. The way that his head is laying on his shell, might be a side affect from whatever he is suffering from. This also applies to him not wanting or being able to eat without lots of help from you. Every animal reacts differently when it is sick, and unfortunately, all of the symptoms he has means he need professional help. I do think that keeping him out of the water, except when feeding ( leave him in shallow water for just an hour or so a couple times a day) That way you don't have to worry about him drowning, because from that horrid, sad, disheartening video, it looks like if he is kept in water that he is fully submerged in, he will drown.
Also, make sure that the temp. of the container you are keeping him in ( the dry container) doesn't get too cold or too hot... don't put it anywhere close to drafts, etc. Other than that, helping him to eat by manually feeding him is going to be the only thing to help keep him alive.
I can tell you really care about this little guy and I wish you the best of luck. I just hope that the vet can do a full work up on him, bloodwork, X-ray, whatever it takes, and that he gets back to normal. ( if the other turtles aren't having problems, it seems like it wouldn't be anything to do with the conditions in the tank. Also, you will probably have to keep him separate from the others for a very long time. Seeing as they bit off his tail, they probably would pick on him again, every after he is better... You might even want to consider keeping him in a separate tank even after he gets better, indefinately. It might be better for him in the long run, since the other two seem to want to pick on him.
I hope you find the cause of all of this and that he re-cooperates completely. I wish you the best of luck!
My prayers are with you both.
Keep us posted on his progress.