First of all, I am getting that it is your child who has some social issues, and it breaks my heart to hear how he is friendless. Now, I will play devil’s advocate and suggest, that as a parent, if I knew there were very specific problems with a particular child (such as bullying, hitting, etc.) I would probably not pick that child as a social friend for mine. If it is more that your son is extremely shy, non-athletic, or a little immature, I would speak to the teacher about encouraging the class to play together as a group and not leave anyone out, so he might be able to develop a friendship, and I would invite the other kids over for a Shabbos party, to do homework together, etc.
My daughter worked as a shadow for a child with Asperger’s-type symptomology, and she worked especially hard at helping him to develop socialization skills (I am not saying this is your situation, but just mentioning it by way of an example). Perhaps your son needs a similar type of assistance to help more fully integrate him socially. Again, I feel how painful this must be for you. No parent can ever stand to see his/her child be hurt like that, and I wish your son hatzlacha in overcoming this obstacle.