How can I play at same time with my son on another tablet?

If you’re just wanting to play while he plays and use a separate account,...

1) you can start a 2nd account and start playing (be sure to link it thru Facebook or Game Center so you don’t lose it if you swap devices)

Or

If you’re wanting to play using the same account...

1) Link the existing account to Facebook, download the app to the 2nd device, open and link it to the same Facebook account. Be sure to “allow progress from another device” when prompted.