Juan 21:21

Peter seeing him Peter therefore seeing him. Once more we see the intimacy between these two Apostles. When S. Peter is told to follow, S. John does so also unbidden; and S. Peter having received his own commission asks about that of his friend. Comp. Juan 18:15; Juan 20:1 [15].

and what shall this man do? Literally, but this man, what?Not so much -what shall he do?" as -what about him?" What is the lot in store for him. The question indicates the natural wish to know the future of a friend, all the more natural after having been told something about his own future. Hence the -therefore" at the beginning of the verse. As usual, S. Peter acts on the first impulse.