When your son is 12, are you still going to need term life insurance?
I think so. If so, you'll want to buy that 20 year term life policy.
It's not actually that common to have a need for a term shorter than the length of time it takes your child to grow up to an adult.
You can by a 20 year term life insurance policy and lock-in a low rate for the entire 20 year term of your policy. You may also want to consider renewable term life insurance which offers you the option of renewing your policy for another term when the coverage ends.
Here's how you can learn more and compare 20 year term life insurance quotes online.