I have not seen any books at all for children about grade retention. If your school system has a psychologist, they may be able to help you, but from experience I have learned that most school psychologist are against retention and try and find other options, but I do understand your perspective. I also have a son who is very small for his age, very immature as well. Until this year he was doing well in school, but now in 4th grade, that has suffered too and I find myself wondering if I should have had him start kindergarten a year later. Does your school offer a 1st/2nd mix class? That might be another option to explore. Then your son could be working academically where he is, but be with kids that are of his size and maturity.
Oh and seeing where you live, you might also check OHVA to see if that is a fit for you and your son.