I would not allow myself to be greatly influenced by what other parents of other twins have done. Make a list of all the possible positives and negatives of both advancement and retention. Consult with a child therapist who may be able to offer some significant input after a few consultations with your son. You know your son better than anyone. What decision would be healthiest for him in both the short term and the long term?
I would also recommend a superb parenting book -- The Joy of Twins, by Pamela Patrick Novotny.