Theres this boy and I really like him, What do i do?

Well hes my age (15) but hes been hanging out with my little brothere aloth abd hes 13 we flirt like all the time but he has a girlfrend whos like my brothers age=[ and shes not even pretty and she cheets on him... but anyways he always asks me like what if i didnt have a girlfriend but i dont know if hes kidding or not cause hes like never that serious except when its only me and him but i have no idea what to do and i think he knows i like him but im not sure...