Encouraging kids of this age to pair off and to act like boyfriend and girlfriend is not advisable. Many parents think it's cute when younger kids mimic the boyfriend/girlfriend behaviors of older adolescents and adults, but it's not appropriate or healthy for kids this age to spend time with each other exclusively.
Ask your daughter what she likes about this or any other boy she finds appealing. It might be fine to ask him and a few other boys and girls to eat supper at your house and play some board games together. Getting your daughter involved in some extracurricular activities that she enjoys will bring her into contact with other kids on a regular basis and give her an opportunity to feel good about herself in ways other than the attention she gets from an exclusive boyfriend. If she is not keeping up with her friends, or is ignoring their overtures to socialize in favor of spending all her time with this one boy, have a talk with her about the value of loyal friends. Let her know she doesn't have to choose between this boy and her friends.
Don't condescend to her or blame her for wanting a boyfriend. Understand her reasons and appreciate her desires, but be sure to discourage any exclusive coupling at this age.