Hi! Welcome to the board!
If your son at 5 can "He adds, subtracts, and multiplies double and triple digit numbers and has an incredible sense of numbers." then going into Gr. 1 will probably not meet his needs and allow him to "learn" something in Math. Check the curriculum and find out for yourself at what grade level the children learn single digit multiplication and division...
As he appears to be highly gifted he probably learns at a much faster rate and would not need any repetition (drills) like normal-average ability children would.
He may not be able to sit through K or Gr. 1.
Your son may have asynchronous development. His mind/ability will be able to learn so much but his output may not catch up. Google for gifted asynchronous development. He may need a scribe to help him output his thoughts. Get him started in learning to type.
It is great that you are here, keep learning more. What local gifted-talented resources are in your local area/state? Private/public magnet schools?
Consider subject acceleration for math ...
Gotta go
Best wishes!
