Then tell him to respect what you come up with, even if it's "not what HE thinks is ideal." Shut him down with information. 24" bull barrel .308AR is gonna be ONE HEAVY PIG that you don't want to carry anywhere - including from the car to the shooting bench.
For 400 yards, you can very easily dothat, accurately,with a 16" barrel. However, throw the FIL a bone here and just get this:
https://www.aeroprecisionusa.com/m5e1-308-20in-cmv-complete-upper