We had our son repeat fifth grade in a new school, and he did much better with the work and responsibility this time around. He still interacts with boys from his "old" school and still says that he wishes he could go back. If he does excel over the next year can he be moved up a grade now that we have held him back, or is it best to leave it alone?
You would be better off letting your son take each successive grade in the appropriate order from now on. Even though your son has caught up to where the others were at the end of last year, they will always be a year ahead in terms of the curriculum. Each successive school year builds on the skills learned the year before, and skipping over a grade could put your son behind again.
If your son really wants to go back to his old school, discuss with him that possibility. Would he be comfortable going back if his friends (and even children who were just acquaintances) are in the grade ahead? If he decides to try that, be sure to talk with the school counselor and his teacher to enlist their support. If your son decides to stay at his new school, make sure that he still has opportunities to be with his old friends.