Upvote:4
He does not accrue "unlawful presence" for the purposes of the 3/10-year bans under INA 212(a)(9)(B) while he is under 18. He will start accruing "unlawful presence" the day he turns 18. If he leaves more than 180 days after that, he will have a 3-year ban.
But not having a ban doesn't mean they will let him in in the future. The immigration officer at a future entry can deny him entry as a nonimmigrant for anything that the officer doesn't like about the person's situation, including any past overstays.