This could be behavioral, but it could also be due to something internal. If he was eating normally before you got him, it’s possible you have just spoiled him and allowed him to develop bad eating habits. At this point, he is going to be very malnourished after eating such a limited diet for months. He also could have high levels of potassium and iron now due to these foods. You should go ahead and take him to an avian veterinarian for a full exam and bloodwork to see where he is at this point. Your vet can then consult with you about nutrition and about not giving into picky habits. However, picky habits can develop due to internal issues, too, and the fact that he is favoring soft foods could be due to a specific problem. I would recommend having him tested for avian bornavirus just to rule it out. Changing homes is usually very stressful for a bird and can result in underlying issues taking over, or can cause behavioral issues to emerge. Because this could be either or both, a Vet check is your first action so that you know how to proceed for getting him back to a normal diet.
Thank you for asking Lafeber,