Answer:
1. Address the Korean national in english, and use proper etiquette - just as you would with any other customer. If he knows no english/there are no other languages you two have in common, either get someone who speaks Korean to translate or use a translator app.
2. Offer any filling food that is halal (no pork, carrion, alcohol or blood products), such as bread or a snack product. If you’re not sure what to give him, ask them what they would like to eat.
3. You need to ask for pronouns first and then use those, or don’t use gendered speak at all in the way you address them. A simple and friendly “Are you having trouble finding anything?” would work.