It depends on your ligaments. If it is the first time, he won't be able to use the shoulder 100% for a month or so. If he has done it a lot, he will probably be fine in a couple of weeks.
wouldn't it be a constant weak spot though, vianaesque ?
Correct, but the healing time will decrease because the ligaments get more damaged the first time you do it. The chance of it to happen again will increases though.
I dislocated my shoulder twice within 2 months in 2008, and the second time hurt far more and took longer to heal for me.
Probably because it wasn't healed totally from the first go-round.