Nothing about your son's the returned 2023 contribution goes on Form 8606. The returned 2023 contribution is treated as never having been contributed. Any gains required to accompany the returned 2023 contribution, shown in box 2a of the Form 1099-R, will be includible on 2023 Form 1040 line 4b.
I assume that your son is under age 59½. The excess contributions made for 2019 through 2022 added to his contribution basis, so the regular Roth IRA distribution of an amount equal to these excess contributions will be a nontaxable distribution of contribution basis, calculated on his Form 8606 Part III (2023 Form 8606 assuming that the distribution is completed by December 31, 2023). This distribution will also appear on his 2023 Form 5329 line 20, eliminating the excess.
