The main "acceleration" option available to 5th graders would be the second year of a "compacted" math sequence in which they complete part of 5th grade math and do all of 6 th grade math. Some elementary schools have this in house at others the accelerated kids go to the middle school for their math class.
There are still kids further accelerated under the old system who have been allowed to continue on their path, but I'm not sure if that is the case for any kids as young as your child.
I'm not sure if they would be helpful, but you might try contacting the MCPS Advanced Enriched Instruction office
http://www.montgomeryschoolsmd.org/curriculum/enriched/
Good luck figuring out something appropriate for your child.