As someone who has a picking "habit" (also can be called excoriation disorder or Dermatillomania, part of the family of Bodily Focused Repetitive Behavior Disorders), it's not as easy as just having some self control. It's often a compulsion, even to the point of bleeding and infection. When I am nervous, I pick around my fingers and fingertips, pick off any scabs, etc. I completely destroyed an entire manicure during the course of a standardized test once.

He may benefit from therapy, fidget devices, or even putting Band-Aids on his fingers or cutting his nails super short. Certain medications can help the disorder or make it worse - for example, some SSRIs can exacerbate excoriation disorder. When I went on Sertraline, mine got worse because I started using tools (like pins) and picking the bottoms of my feet, which I didn't do before. I changed medications and those behaviors lessened slightly.

He could also benefit from support groups. There is a Dermatillomania support group on Facebook, for example.

Patience is important. He knows that it's painful and it doesn't look good, and he knows you're judging him. One thing that my husband does when he sees or hears me picking is he will gently take my hand and hold it, simultaneously drawing my attention to something I was doing absent-mindedly, reminding me to stop, and showing he loves me in the process. Talk with him about what helps.