If my friend says to me, "May Allah bless you," how should I respond to him?

If your friend says to you, "May Allah bless you," you can reciprocate by saying, "And may Allah bless you too," or you can offer another supplication for him, based on the Prophet’s (peace be upon him) saying: "Whoever does you a favor, then repay him. If you cannot find something to repay him with, then supplicate for him until you think that you have repaid him." This includes both verbal and physical acts of kindness.