Budgie is losing feathers from all over his body.

Hello!
My name is Sujoya, and I'm a mother of 4 budgies. One of my Budgie who is 1 year 6 months olds is losing feathers constantly.
Firstly, when I saw bald patches.. I assumed it was moulting because He was having pin feathers. But soon I got to notice the bald patches is increasing, the pin feathers grow and turn dark and fall off. It's been 6 months now and he has lost a lot of feathers.
I took help of Avian Vets nearby me, and they told me different reasons for it.
My little budgie is scratching all the time, I do give him good diet food like seasonal veggies and vitamins& calcium. He eats well, sleeps well and plays too. I'm concerned about his feathers, and no where I could find the solution.

I tried Original Alovera application, haldi application, a light dettol water spray as the doctor told me, I used the spray products for mites, neem spray and nothing is working.

I never keep my budgies in cages, they're always free. The kind of sellers here in Ranchi, Jharkhand clip off their wings. It's so sad, That's a major reason I never keep them locked and they all are very friendly.

Please Help me out, I'm very worried about my little tweety bird. I love him so much, most of my time is spent with him. Now, I've started to notice My other budgie has also started to lose feathers .. I'm very Worried please help.