Is there any chance he's leaving his tail feathers in the water dish?
Yeah, something isn't quite right if his tail feathers smell like poop...either he's got them in his water dish, or he's spending a lot of time on the bottom of his cage, which may not be getting cleaned-out often enough if there is that much poop on the bottom that it's getting all over his tail. And if he actually has poop stuck to the underside of his tail or around his vent area, that is typically a sign of illness...Do you know how he's getting his tail in poop all the time?
I'm not trying to be accusatory towards you, but rather just want to make sure your bird is okay and his cage is getting cleaned often enough (once a week is a minimum cage cleaning interval, more is better)...It's not normal for them to "smell like poop", they typically don't smell like anything at all, except for their normal "birdy" smell...
As far as washing his tail-feathers in something, first you want to find the root of the issue so it doesn't keep happening, and if he actually has poop stuck to the underside of his tail or especially around his vent area, then this is going to require a visit to his Certified Avian Vet, as usually this is indicative of some kind of GI infection, either bacterial or fungal, but you can't know without a fecal culture being done.
Once you figure out why this is happening, if you want to wash his tail feathers, the best thing you can do is simply give him a bath in warm water and submerge only his tail feathers that are dirty/smelly. They do make a couple of "cleansers" for birds, but they are hard to find, and frankly I don't trust them all. There's one that is made out of nothing but natural orange extract and some plant enzymes, it comes in a pump bottle and you can buy it at Petco, but I don't remember the name of it, as I only bought it once and then didn't like it, as it left a heavy film on the feathers that had to wear-off, so that's a no-go...You're best to just wipe him down, only concentrating on his tail feathers, and use JUST A SINGLE DROP of regular, blue Dawn dishwashing liquid. Nothing else, no other scents or brands, and this is only to be used on his tail feathers. Use a spray bottle or something similar, fill it with warm water, and add one drop of the blue Dawn dishwashing soap. Then mix it up, soak a rag in it, wipe down his tailfeathers only, then rinse the tail feathers very, very well to get the soap off...And never submerge him in a bath of Dawn...it's safe, but I hate to use soaps on my birds...
