My Quaker parrot keeps biting his wing

I've had my quaker parrot for over 3 years. He is a male bird. Recently he has been taking a liking to biting his wing. Basically, he bites a specific part of his Wing and then starts chewing whatever he takes out. The top inside of his left wing is starting to look bald. Is there anything I can do to prevent him from continuing to do this? Right when he starts doing it, bites me if I try to interfere(he rarely bites me viciously). Is there a reason for why he could be doing this?

Notes:
-He does masturbate.
- He seems to start biting his feather the most when he's about to put out his food. What he does is this
1. Starts fluffing up his feathers

2. Then he starts to look like he is reurgitating his food.

3. Then he starts biting his wing and then takes a piece of the inside of it and starts chewing it.
- I let him out every day for a few hours. obviously i cant let him out all day due to school.
- He is given a a seed/pellet diet, with fruits every week or two.
- He doesn't bite any other part of his wing. Or even pluck his other feathers. It's just this part. And he always seems to do it the most when he's about to masturbate, or when he is about to spit out his food(in a way to try to feed someone)
- He is given a shower 1- 2 days out of every week
Other than this, he's never really had any health issues.

You need to get him to an avian vet for blood work and possibly an X-Ray to rule out medical problems. It could be due to hormones, enlarged organs, heart problems, or something else.

After health problems have been ruled out, then you can work on behavioral issues.