I agree with OHGrandma. I'd have him start as a Tiger. He wouldn't be able to bridge to Boy Scouts with his den if you put him in a den with older boys now.
Here's the age info from the BSA site.
"Cub Scouting is for boys in the first through fifth grades, or 7 to 10 years of age. Boys who are older than 10, or who have completed the fifth grade, can no longer join Cub Scouting, but they may be eligible to join the Boy Scouting or Venturing program.
Boy Scouting is for boys 11 to 17 years of age. Boys also may become Boy Scouts if they have earned the Cub Scouting Arrow of Light Award and are at least 10 years old or have completed the fifth grade and are at least 10 years old."
