This behavior can be caused by a ton of different issues, but since Kiwi isn't having loose or abnormal stools, no blood in his stools, no changes in the color or texture of his Cere, and is not at all acting like he is sick, i.e. no lack of appetite, no fluffiness, no lethargy, etc., then most of the very serious causes can be ruled out, especially if this has been going on for a couple of months.
The fact that he has some small downy feathers on and right around his vent is normal, yes, but it is possible that fecal matter can get stuck around the vent in those feathers, causing itchiness, yes, but not for a couple of months. If this was going on for a couple of months you would have noticed the fecal matter matting around his vent, and typically when that happens for that long a period of time, there is a serious underlying health issue and other signs/symptoms of illness, so I doubt that is the problem.
They do just simply get areas/patches of dry, flaky, itchy skin, as well as have skin allergies. I can't tell from the photos really, but it doesn't appear that his skin is flaking or dry, nor is there any redness or swelling of the area, which is odd, and is indicative of an internal vent issue, but could still be an allergy.
Did anything in his diet change around the time that this started? How about any changes around that time period inside of his cage or on the common places where he typically spends his time/sits on? Any new perches, new toys, anything that his vent area comes in-contact with on a regular basis that's different? Different laundry detergent? Their vent areas are technically bare, at least right directly around the vent, so it is possible that when they sit/perch/lay on anything that the skin around the vent comes in-contact with whatever they are sitting/laying on, so you have to brainstorm and think about where he commonly sits/lays, and what may have changed around that time in your house, or in his cage or his "spaces"...
Have you tried gently washing his vent area with an antibacterial soap, patting it dry, and then immediately applying something daily like all-natural aloe vera, or Hydrocortisone cream, or Benedryl cream? I would definitely try this, at least once or twice, and see if it brings him any relief. This most likely isn't a cure for what the issue is, but may bring him great relief and keep him from over-preening and scratching the area. And using any of the above creams will not cause any harm, so why not try to bring him some relief...
This may very well be a case of mites or worms, as the number one sigh/symptom of GI worms is constant vent scratching. It's most likely not mites because he would probably have them elsewhere too, but it could very well be worms or some other GI parasite, which will easily be diagnosed by your Avian Vet by him taking fecal swabs.
The other thing it may be is a GI Fungal/Yeast infection, or a GI Bacterial Infection...Do you give him Probiotics? You may want to start doing so, as again, they will do no harm at all, but do often help all the way around. And any time your bird is put on any type of Antibiotics for a bacterial infection, you should always put him on a course of probiotics during and at least directly after they finish the Antibiotics, as it's very common for Antibiotics to kill-off all the healthy, normal bacteria in the bird's GI system, which is what keeps fungal infections, specifically yeast, at bay. So it's very common for birds and also reptiles like Bearded Dragons to develop GI thrush or other fungal infections secondary to them taking a course of antibiotics for a bacterial infection.
Again, fecal swabs will confirm or rule-out any bacterial and/or fungal infections that may be causing the itching, but in the meantime, since you have to wait over a week to get him to the vet, I'd pick-up a bottle of avian probiotic powder and start giving it to him daily, for 2 different reasons, first because he may very well be suffering from a budding yeast/fungal infection and the probiotics will help to kill off the fungi, and secondly because if he ends-up having a bacterial GI infection that is causing this instead of a fungal infection or a parasite, he will be put on a strong, broad-spectrum antibiotic to treat it, which can very well cause a secondary fungal infection, so giving him a daily probiotic supplement for the week prior to possibly starting a course of broad-spectrum antibiotics will up his normal/healthy GI bacteria count and protect him from developing a potential secondary fungal infection. Petco sells a great Avian Probiotic supplement, it's a pretty large bottle of powder with a dose-size scoop, and you simply add it to his food once a day, and it costs $9.99. I'm a believer in probiotics, prebiotics, and natural medicine/supplements, and I have been giving all of my birds a daily probiotic for years, along with the baby budgies I used to breed, and none of them have ever had any type of bacterial or fungal infections, not ever, and never had a baby develop slow crop or crop stasis. Prebiotics and probiotics can not do any harm, they can only help, which is a good reason to at least try them...
While waiting for his vet appointment I would again cleanse the area gently with an antibacterial soap and warm water, pat it dry, and then apply either all-natural aloe vera, hydrocortisone, or Benedryl cream, this will give him tremendous relief and would also help if he happens to have some kind of dermatitis back there, or something like Feather Cysts, which i doubt in the vent area, but is possible.
Make sure you are checking his droppings every single day for looseness, mucous, paleness, or any sign of blood, and report this to the avian vet. Also, watch him to make sure he isn't scratching or frantically preening any other area of his body as well that you haven't noticed.
I would feel terrible for writing it off for so long (I already do as it's now gotten worse). Where on earth would he have picked up a fungal infection or worms over the winter though? My husband and I haven't been sick with anything, and certainly not with fungal infections or worms! He went for a regular checkup in Novemeber and wasn't in the outside world again until starting sporadically last month as the weather began warming up. I started noticing the increase in preening near the butt in maybe late Feb. early March...
