Rex Lestrange shakes Tom Riddle's hand as if the boy is just as much an adult as the older student. It was designed, Tom supposes, to make him feel obligated to the pureblood family, but all it does is give him yet another opportunity to show his superior breeding.
Rex is slightly surprised to find Riddle's grip as firm as his own, and even more surprised to see that he greets his parents with a bow and their proper titles.
It seems the young student was not entirely isolated his first year at Hogwarts, as he thought.
It makes Rex wary of the boy. He does not stop watching Tom Riddle the entire three weeks they spend in France – and he is sure the younger wizard knows it, too. How, otherwise, could he act so perfectly?
He is perfectly grateful for the kindness – even though he must know it is so much more than simple kindness, he is a Slytherin after all – and he addresses the entire family politely, even when Rex's eight-year-old sister begs the boy to call her "Ronnie".
He asks all the proper questions – about genealogy, about politics, even about the rising Dark Lord and his relation to the Muggle violence in the East.
Wary he may be, but Rex is also impressed. He can tell his parents are, as well, by the way they treat him – not as a son, not quite, but as something of their own little protégé. He will do well by their family, if only they can train him up right.
First, to do something about those horrid Muggle relations of his. Highly-blooded as the Riddle boy is, on his mother's side, the Lestranges know his father is naught but a worthless Muggle. He would never make it in pureblood society without someone backing him, and of course they are willing to take up the mantle – Riddle will do great things, they know.
And a Parselmouth, as well. That snake of his is loyal as no other pet could be – yes, the Riddle boy will do quite nicely for the Lestrange family, once certain problems have been fixed.