Your son likes to play with this little one, maybe

because he is more easily taken advantage. But if I were his mom, I would even be concerned that your son is in the same class at school. His son is 1/2 yr younger. Sometimes the younger one is simply afraid of the older one and forced to some agreement, such as exchanging toys. He may not be really willing to do so. Has the exchanging toys idea always been initiated by your son? If that is true, I would not let my son play with your son too much. Especially you did not realize the situation is unfair for the younger one.