Different sources I have seem to look at this differently.
Robertson suggests "when he is come", true to his punctiliar
understanding of aorist. I think I understand that.
BDR also seems to see aorist as punctiliar, and gives three cases:
(1) the beginning is stressed(EBASILEUEN, became king), (2) the end
is stressed (EBALEN, met), or (3) treatment as a whole without respect
for time. I don't know how to apply this paradigm to the above passage.
Young's book emphasises this third use from BDR: aorist as a helicopter
view, seen as a whole, not as repeated action. I'm not sure if I understand
this or not. If I do, then ELQWN would mean that a change of state has
occurred, and this change is seen as a whole.