Welcome. Cute birds! I have a Lesser as well, and he is wonderful - just the happiest little guy with a funny sense of humor and an major attitude. He plucks his feathers a little bit seaonally. Nothing too bad - just his legs.
Our M2 started plucking his chest after an illness. They said he was all clear, but the plucking as continued. We are not sure if it is behavioral or if there is still some medical issue.
Plucking can be caused by an internal or external (skin) illness, allergies or just behavioral patterns. It sounds like you've brought him to the vet, but you may want to check for other medical issues. Common food allergies can be caused by peanuts, colored dye in pellets, and grains. I'm sure others will help me out. If you feed peanuts or peanut butter, that would be the first thing I'd stop. Almonds are a better choice, and almond butter is a healthy substitute for peanut butter. The 2nd thing would be switching him to a different type of pellets. Easier said than done - many cockatoos (including mine) are difficult when it comes to switching foods.
It's great that you are able to get him out, as sunshine is one of the best things for a parrot! Remember that even a clipped bird can fly. My CAG was out overnight once, and we were so lucky to get her back. My Harlequin Macaw also ended up 50 feet up in a tree once. He is not a good flier and was lightly clipped. Fortunately, I saw where he landed and we were able to convince him to climb down with a jar of pistachios, since there was no way he was going to do that horrible flying thing again. He climbed to the END of each branch until it bent from his weight, and then got onto a lower branch, went to the end of that, etc. He finally got low enough so that we could reach him. Anyhow, I bring them out in travel cages or harnesses now! I'd hate to see you lose one of those cuties!