There is a ban in INA 212(a)(9)(B), where if one accrued 180 days of "unlawful presence" and then leaves the US, they trigger a 3-year ban; if one accrued 1 year of "unlawful presence" and then leaves the US, they trigger a 10-year ban. "Unlawful presence" usually starts accruing when one stays past the date on their I-94. However, for the purposes of this ban, "unlawful presence" is defined such that one does not accrue unlawful presence while under 18. So if your son is under 18, he is not accruing "unlawful presence" right now, even though it is past the date on his I-94. If he continues to stay in the US, when he turns 18, he will start accruing unlawful presence. If he stays past age 18 + 180 days and then leaves the US, he will have accrued more than 180 days of unlawful presence, so leaving the US will trigger the ban. If he leaves the US before age 18 + 180 days, he will not have accrued enough unlawful presence before leaving to trigger a ban.
But like I said, not having a ban does not mean they will let him in. Any overstay could increase the chances they will not let him in if he leaves and tries to come back.
No. Generally, he is not eligible to apply for Extension of Stay if his status is already expired (unless there is really good cause to explain the late filing).