How?
It is clear nepotism. He is a decent player, and good luck to him, but for the season he has had (solid / decent, but not standout) there is no justification for (a) his selection in the squad over other better performing players and (b) the amount of minutes he got in the World Cup, again ahead of players who have far more of a justification for being selected. He did well in the World Cup, and credit to him for that, but he was given the opportunity to do so above others who deserved the opportunity ahead of him. That fact that he was given that opportunity ahead of others more deserving, and his father is on the coaching staff, is nepotism.
The situation should never have arisen. If Popovic was considering Okon-Engstler, Okon (Snr) should have stood down from the coaching staff, or more importantly, FA should have forced that to occur.