Hi
To some point yes you have to be born in Philippe Stern's family to be his son.....generally. 
Then after 19 years working for Patek Philippe I guess Philippe Stern has had the time to see his son show leading and managerial capacities, to prove his value for this position.....
Then that's their decision and those who know or I've met Thierry Stern can only congratulate him IMO! No worry! 
Cheers
Dje