Well, first he lays off the guitar for a while - he needs all the fingers on his left hand to make music (or at least to attempt it, in my case).
Then he probably picks up the guitar prematurely, when the healing seems to be complete. But playing the guitar involves pushing a tightly stretched, very fine wire against a block of wood. Without a callus on the fingertip, this can be painful. On a freshly healed cut, it results in blood on the fingerboard when the cut reopens.
Then there is more waiting. And while there is waiting, the calluses on the other fingertips, an adaptation by the body to playing the guitar, are slowly going away.
Finally, tentatively, he tries again. But now, tho his fingers know what they need to do, they are soft and weak, like those of a guitar virgin.