I think it would be best to actually contact the school you are interested in sending him and get their input. You don't say what issue came about with him in third grade. Is he not able to keep up academically? There's a learning center in Baxter Village in Fort Mill that could possibly help him adjust to a new school and his studies. I don't think you'd have to sign any contract. You could have him there just for as long as you need. Here is a link:
NewPoint Learning Center | Tutor and Tutoring Services | Fort Mill South Carolina SC .
I'm a summer baby myself, but have been out of school for a loooooong time! :-)
Get your son's input, too. Maybe he won't want to be held back and might do really well at the center. I don't know how much it costs, so that may be an issue, I don't know.
Whatever you do - good luck! I'm sure things will work out nomatter what you decide. Hang in there!