Sympathy, bloss, I have a picker too. As you say it is practically impossible to stop them. I have tried the very short nails and it does work briefly but IME the nails grow back so fast there is enough to scratch with again in a couple of days.
Does the thing itch or is the picking just a habit? Could you bribe him? If he only does it in bed in the morning could you tell him that every day he comes straight in to you without picking, then he can have a reward later - only a little thing obviously, sweets if necessary?
Have you tried calendula cream on it? Whenever I have something that won't heal, not from picking in my case (honest!), eg those cracks in the tips of fingers, I put calendula on at night and then a plaster and it makes a big difference just overnight.
Good luck!