How do you say? I really miss you
I would like to know how to tell someone I really miss him
I'd say, "Te extraño mucho." You could also say, "Te echo de menos."
To clarify slightly, in the United States and in Latin America one would be more likely to say "Te extraño mucho"; however, in Spain the verb "extrañar" generally means "it seems weird", so we would say "Te hecho de menos mucho."
Ej. "Me extraña mucho que tu perro sea tan grande." = "It seems weird to me that your dog is so big."
I have heard the frase "to me faltas" in a spanish song". I always thought this could be translated as " I miss you" but now i'm not sure anymore.