"Good luck!" "Go for it!" "Show 'em what you can do!" Seriously, why not say "¡Ánimo!¨and explain it? Everyone likes picking up bits of other languages, "¡Ánimo!¨ sounds pretty cool, it will sound sincere, and it will be different from the encouragement other people are giving him.

I think a good phrase would be "cheer up!" or perhaps "keep it up!" "Cheer up" is good if say, a friend is looking dejected and you are trying to improve his mood. "Keep it up" is used in situations where someone needs motivation or encouragement to continue their current task.

"Go for it!" "You can do it!" "Don't lose heart"