Probably OCD or maybe habits created by situations but this is the most I can guess. I used to bite my nails and then moved on to biting the skin around the nails but eventually I stopped and now I don't do it no more. The only thing I have is scalp picking and I hate it. I pick and pick scabs till I bleed. But the thing is that I get bullied a lot and even at home the people I live with treat me bad (But not my parents) so I feel like c**p. So that's why I'm saying that these habits couldve been caused by some past situations or something but idk. So i would recommend if you ask him if hes passed any bad times or something because it could be that, Cause of all the stress n stuff but yeah. Best of luck to you and I hope you can resolve this! :)
I usually eat my boogers because I dont have a tissue around and its a good way to get rid of them instead of wiping them on the couch.
Something about when it sticks to the back of my teeth makes it irresistible!
Also the feeling of ripping an old crusty scab off is simply liberating. And then if it has a good crunchy texture, you'll be finding yourself picking up contact sports just so you can pick your wounds later!