Question about aspect: why is the first imperative present and the second an aorist? Is there a real distinction (that escapes me), or are the two often interchangable and chosen for for instance metric reasons?
I think the present imperative has a sort of dramatic effect here, not simply “go!” but “be going!” As always, a present stem implies a continuous, undelimited action and an aorist stem a complete action. So καλεσσον as an aorist shows that calling Aias is a whole, complete action.
I would say that the form might be chosen for metrical reasons, but that also changes the meaning a bit. I suppose an aorist form might have well been used instead of ερχεο, and then it would mean “go!”.