Welcome!
Just a few random thoughts:
1) Can you arrange a playdate with your child's friend? Possibly the families could go to a park, museum, kid-friendly restaurant, etc. While the children play or enjoy each others company, it would be your mission to learn whether your son's feeling of friendship is reciprocated by the other child, and whether the friendship is welcomed and supported by the parents.
2) If the friendship is reciprocated, welcomed, and supported, you may wish to ask whether the preschool can group the two children together... whether your child moves back to the old group, or whether the other child moves to your son's group.
3) If the daycare/preschool is open to donations of any books or toys, supplying some which are of particular interest to your child may act as a magnet for children with similar interests.
4) Books such as "The Unwritten Rules of Friendship" may be of interest to you as a parent, for coaching your child if you perceive that he may need direct teaching about building and maintaining friendships.
