A photo of the entire tail/entire body is necessary here to access whether it's infected, and whether it may be spreading. IT MAKES ABSOLUTELY NO DIFFERENCE HOW CLEAN HIS TANK IS, BACTERIA LIVE EVERYWHERE AND A WOUND THAT SEVERE IS A BREEDING GROUND FOR INFECTIONS, ESPECIALLY IF THE BONE IS AT ALL EXPOSED AND/OR WAS FRACTURED!
Any injury that results in an open wound AND a damaged underlying bone REQUIRES A CERTIFIED REPTILE VET IMMEDIATELY!!! Had you taken him when it happened they would have not only cleaned, debrided, and dressed the wound (and possibly either set the tail bone or amputated above the wound/fracture), but they would have most likely given him antibiotic injections AND put him on oral antibiotics as a precaution. And the reason they do this is because a lizard's tank is a breeding ground for infections, regardless of how clean it is kept!
Forgetting that this has happened before to the same lizard (that's upsetting and suspicious), he's in a lot of pain even if he isn't outwardly acting like it in your opinion. Any time a bone is damaged or broken it is extremely painful to any living creature. So he also needs some prescription pain meds from a veterinarian in addition to the antibiotics. Even if you think you can treat a serious wound/fracture by yourself with a makeshift cast, you need to think about the pain he has been in since it happened. And if that doesn't get you to take him to a certified Reptile vet ASAP, then the extreme likelihood of "Tail Rot"/Necrosis/Sepsis should be getting you in the car with him right now!
"Tail Rot" is a general term, not an actual disease in and of itself. To sum it up for you, the tails of lizards commonly become infected as a result from either a serious injury, such as this example, stuck shed, etc. because #1 their environments are perfect for bacteria to thrive, and #2 because the blood supply to their tails is not great, therefore their tails don't heal quickly or efficiently die to a lack of white blood cells, platelets, etc. getting to the area, ESPECIALLY IF THERE IS BONE DAMAGE INVOLVEMENT, AS THERE IS A BLOOD SUPPLY INSIDE THE BONE THAT WILL CARRY THE BACTERIAL INFECTION UPWARDS! As a result of a bacterial infection setting in, the tissue BELOW the wound dies, or becomes NECROTIC. Any blackening of the tissue is NECROTIC, or dead tissue. If the lizard doesn't get aggressive antibiotic treatment with the APPROPRIATE ANTIBIOTICS (an experienced reptile vet will do a Culture and Sensitivity to diagnose the type of bacteria and which antibiotic will work to kill it), the infection will travel UP THE TAIL, TOWARDS THE BODY. The necrosis, or tissue death, will spread UPWARDS TOWARDS THE BODY as well, killing all of the tissue along its way. If the lizard still doesn't receive proper treatment with antibiotics and amputation above the NECROTIC tissue/bone line, the infection then spreads into the lizard's bloodstream. This is called Sepsis, or a Systemic Septic Infection. At this point it's extremely difficult to treat the Infection, and typically the lizard dies a horrible, painful death. Now, in your lizard's situation, where there is exposed, damaged bone, the Infection will spread up towards the body and into the bloodstream much more quickly, because the bone has its own blood supply, and the bone will become NECROTIC and die right along with the tissue. So to sum this all up, YOU NEED TO GET YOUR WATER DRAGON TO A CERTIFIED REPTILE VET IMMEDIATELY, LIKE YESTERDAY. NOT BECAUSE THE TAIL MIGHT BE FRACTURED, NOT BECAUSE IT MIGHT NOT GROW BACK, BUT BECAUSE EVERY HOUR THAT GOES BY WITHOUT GETTING HIM ON APPROPRIATE ORAL ANTIBIOTICS AND MOST LIKELY ANTIBIOTICS BY IM INJECTIONS AND AN AMPUTATION ABOVE THE WOUND, YOU'RE MAKING HIS TAIL MORE AND MORE DIFFICULT TO TREAT.
Trust me, you'll end up spending a hell of a lot more money if you wait to get him to an experienced reptile vet any longer. Typically what I see in this situation is the owner refusing to take the lizard to a Herp Vet because they don't want to pay for it, so they use Betadine and Raw Honey and Neosporin and Colloidal Silver etc. and they think they're fine because the lizard is still eating and acting normally. Then one day the lizard stops eating, stops moving and becomes extremely lethargic, and they start trying to force feed it and they still refuse to take it to a Herp Vet. Then they finally break down for one reason or another and they finally take the lizard to a reptile vet, and what would have been a bill for an office visit, an antibiotic injection, a round of oral antibiotics, oral pain meds, and possibly a small amputation that amounts to between $100-$200 is now well over $1,000 because the lizard has to stay in the hospital for tube feedings, subcutaneous fluid injections, IV antibiotics, treatment in an oxygenated brooder, and actual serious surgery to remove the entire tail along with tissue in the lower abdominal area. And over 50% of the time at this point THE LIZARD DIES ANYWAY! Happens ALL THE TIME! I'm tire of seeing it, so I'm not pulling any punches anymore. What I just wrote can be verified by searching for "Tail Rot" or "Broken Tail" and reading the hundreds of posts that I just summarized. It's so preventable, not expensive if you just act like a responsible pet owner and take him to an experienced reptile vet now rather than later.
Take your pet to the reptile vet immediately. Stop playing doctor and do the responsible thing. If you or your human child had this same thing happen to a finger, toe, hand, foot, etc. you'd be in the ER within the hour, if for no other reason than the pain. You need to think about that.