When Should the Bar Mitzvah Boy Wash Mayim Achronim

If we tell him to wash before shkia, the Mishnah Berurah (181:24) writes that one should be careful not to interrupt between mayim achronim and bentching even with divrei Torah, and if he we tell him to wait until after tzeis hakochavim, perhaps there is only a heter to wash mayim achronim on Tisha B’Av for one who needs to eat on Tisha B’Av, however, who says if one waited until Tisha B’Av when he could have washed earlier that there is a heter? Seemingly, the best thing to do would be to make himself obligated in netilas yodayim just before he recites birchas hamazon, either he could go to the restroom then, or he could daven Ma’ariv straight after bentching, and then he would anyway need to wash his hands.