For example, you are a 20 year old male/female. You are walking on the street, then you see a male stranger who seems to be the same age as your. He is in hurry, you see him drop his wallet. You pick up the wallet and want to give back the wallet to the person. He is walking quite fast and the street is quite crowded, so there is no way to approach him and pat him saying "对不起，你的钱包掉了" (excuse me, you dropped your wallet). You don't want to let him out of your sight, so you decide to shout to him "sir! you dropped your wallet!". How to say that in Chinese?
Can we use 同伴 or 同学 to address him? (eg: 同伴！你的钱包掉了！)