Hmmm, maybe the definition of 'soft food' you and I are using is different because all parrots love soft foods to the day they die [mine get it every single day for breakfast]. I think that you may be referring to formula while I am talking about food that is soft and wet [things like gloop or oatmeal/pastina/polenta/couscous/steel cut oats mixed with baby jar food] so, correct me if I am wrong but it would seem to me that he went from formula to adult food -which is what he is eating now. ~And the pellets he gets do have soy.
Now, free-feeding any protein food and, most especially, food made with soy is never a good idea. Parrots in the wild don't ever find an abundant source of high protein all year round and, because protein is needed for procreation, growth and life, nature gave them a craving for it so, when they have it, they will always eat too much of it and too little of the healthy stuff. It's the way they are made. If you feed a natural seed eater [like a canary or a finch] and a parrot the same food: a protein one [pellets, seeds, nuts, nutriberries, avicakes, etc], a leafy green and a fruit and/or veggie, the parrot will go for the protein first and then eat a bit of the fruit; but a natural seed eater will go for the green first, then the fruit and last the seeds. Soy is a different story... Personally, I don't feed any of my animals food made with soy because it's GMO [ALL soy is] and it has goitrogenic and estrogenic side effects. But, in all honesty, even if it was not a GMO and had no side effects, just the mere fact that no bird in nature consumes soy [it's toxic in its natural form] would steer me away from it. There is a reason why all the good dog and cat food contain no soy whatsoever...
The reason why I asked about his diet is that baby birds that are free-fed high protein soy foods mature earlier. And, if, on top of this, the bird is kept at a human light schedule, its endocrine system is even more messed up by it so, although this bird is not at an age to be showing any sign of puberty, it might very well be starting it due to the diet and light schedule [which is not a good thing].
But, without being there to observe the bird and the interaction and/or attention it's getting from the chosen human [nobody else really counts for them] and going only by what you tell me, I would say that the problem is that the bird is clipped [can't get to its human on its own] and it's not and possibly has not been getting enough attention from its chosen human. Babies are very mild-mannered and resign themselves to lacks but, once they start growing into their own, they start demanding and I think that's what's happening here. Be careful how you deal with this because the next step for a bird that is not getting what it wants is to start biting. Duskies are very similar to GCCs, maroon-bellies, etc. They need a very low protein diet with a lot of fruits and A LOT of personal attention -and by that I mean the bird ON its chosen human, not in the same room and looking at him/her from a distance because, as far as they are concerned, that's not 'personal attention'. The duskies are aratingas but the little ones behave like pyrrhuras in terms of the attention they need...People seem little and tend to think that they are less trouble than a larger parrot but it's not true, they actually need much more attention than most other species. In terms of how much one-on-one they need, I would much rather have four amazons than a single little conure...
As to putting any bird in a basement... well, unless it's one of those 'semi-basements' with normal size windows, it would not really work out because although you are correct that the fluctuation of daylight hours is what 'tells' their endocrine system what season it is [courting, nesting, breeding, molting, resting, etc], it's the exposure to dawn and dusk that sets their internal clock and not the turning on or off an artificial lightbulb. A basement would work out great as a sleeping 'room' for them because it's nice and dark and quiet but it requires the human to get up at dawn to bring the bird upstairs where it can be exposed to the special light spectrum that only happens at these times -and that means getting up at 4 am in the summer [I am getting up at 5 am now but I will end up getting up at 4 am by mid June
]. As to UV lamps... I don't recommend them. What I do recommend is a good quality full spectrum light [CRI 93+ and Ktemp not higher than 5000] because they produce enough UV A for them to see but not enough UV B to hurt their eyes [UV lamps that produce enough B for the bird to produce its own vit D3 need to be too close and end up hurting their eyes].