I see why you're concerned about this, and it's always a good idea, especially with birds, that when something worries you, you get them to their CAV ASAP, as I always say "better safe than really, really sorry"...So if Taco hasn't had any routine blood-work or a Fecal Culture done within the past 6 months, I would recommend it, just to make sure that nothing is going on, like a Bacterial or Fungal Infection, some type of Renal Infection, etc. You know how they hide all outward-signs/symptoms of illness, injury, pain, etc. for as long as they can, so better to rule it out or confirm it ASAP than to wait...and it's good to have blood-work and a Fecal Culture/Gram-Stain done at least once a year or so anyway...and it will make you feel better too...
As far as the feathers themselves, if he's in the middle of an actual molt (as both my Senegal and my Green Cheek are right now, feathers everywhere!) then it's most-likely him just pulling out the feathers that are coming out anyway, causing itchiness, dryness, etc. The single-feather you found I wouldn't worry about at all unless you find more (and I suggest you keep it too, take it to the CAV with you), and as far as the damaged feathers on the back of his neck, I don't know if you have any other birds that preen Taco, if so then I'd blame that, but if not, then I agree that it's most-likely that he's molting and he's itchy, and he's using his feet/claws to scratch...It could always be the start of some Destructive-Behavior, but again, because he's actually starting his typical, yearly molt, then I wouldn't jump to that unless it continues to get worse and doesn't stop after the regular molt is done...Again, a CAV trip will answer all of these questions for you pretty quickly...
Just a note, I don't know the experience-level of your CAV (or even if you use an actual Certified Avian Vet, or if you have an "Exotics" Vet that he sees), but if Taco doesn't usually see an actual Certified Avian Vet, you definitely want to find your closest CAV and make the appointment with them for this issue...I hate to say this, but it's true and it will save you money and a waste of a vet trip...probably 95% of all "Exotics" Vets have absolutely no experience or expertise in Feather-Destructive Behaviors, nor would they know the difference, and most wouldn't even know what diagnostic tests are warranted when it's suspected. There are a few, very few, Exotic's Vets with the same Avian education, experience, and expertise as a CAV, but they are few and far between, and they are also typically "Exotics" Vets who have themselves owned parrots for most of their lives, and they tend to work in an Exotic's Hospital and see only birds...kind of like "Dr. K" down in Fort Lauderdale, FL...She and the other female vet that works in her practice are the best example I've ever seen of two "Exotic's" Vets who actually have just a ton of experience and expertise in many different types of animals, and Dr. K. is actually a CAV, so that really doesn't apply, but she's a CAV that treats many Exotic animals (no dogs or cats) and who knows her stuff, and won't treat a species that she isn't comfortable with, like reptiles, she usually passes the reptiles and amphibians to the other girl, who is a CRV....
Anyway, I got off-track there, sorry, bottom-line is make sure that you find a CAV with much experience in Feather-Destructive Behavior, as it really is an individual field of Avian medicine all it's own, and that requires experience...