Jody,
I have read and been told that 5 mg/kg/day is the "standard" therapeutic dose. (I say "standard" because there aren't really any established standards for cyclosporine and bone marrow failure diseases.) It is thought that if you are above that dosage, you will have more side effects than therapeutic benefits. Here is a good article comparing cyclosporine alone to ATG/cyclosporine. If you read it, you will note that the intention was to stop the cyclosporine-only group cold turkey at 6 months. However, if their counts were still increasing, they continued them on the cyclosporine at the therapeutic dose of 5 mg/kg/day :
http://bloodjournal.hematologylibrar...93/7/2191.full
Typically, cyclosporine trough measurements and creatinine/BUN tests are done to check that the cyclosporine dosage isn't too high. If you also have access to those numbers (along with your son's current weight and dosage), that would be beneficial.
In summary, I think the thing to determine now is whether your son is on too high a cyclosporine dosage. (It isn't too low, as he has had a good response.) You also want to come up with a game plan to extend the cyclosporine beyond 6 months if his counts are still increasing at that point.