It says: "When attacking", does this mean I can choose to wait until after my opponent has rolled defense dice to reroll with Han?

No.

Order of attack goes:

Attacker rolls attack dice

Defender modifies attack dice

Attacker modifies attack dice - this is where Han's re-roll happens

Defender rolls defence dice

Attacker modifies defence dice

Defender modifes attack dice

Han's reroll ability is considered a modification of the dice by the attacker and must perform this modification at the appropriate time - as per Funkleton above.