To reduce serum in a classical medium you need to pick the right medium. GIBCO has OptiMEM 1 and several Advanced formulations such as Advanced DMEM:F12. You should be able to reduce the serum in these media to 2%. If you prefer to use a published claasical medium consider alpha MEM and supplement with ITS. You would still need to add the 2% FBS (hopefully qualified for MSC's) and of course B-FGF.