Hey,
Is the neighbour's daughter too of his age or elder! In either case, do not stress that much. I guess, the best way to make him get over the infatuation is to ignore it. When we give more attention to such things, kids think that they are making us laugh about the whole thing and they do it more. More so, he is just 8-year-old. It is just a phase. Children get infatuated, even elders do, so it is all okay.