mct16:
"Fiston" is the French for "Son", but like the English version it can simply be used to address someone who is much younger than you are without necessarily having any paternalistic feelings towards them.
Ah... Hmm, well, I guess that could be up to personal interpretation, then? Same thing with the tu/vous?
At any rate, I was thinking about this some more today, and realized that there's another reason reason why I see such a familial relationship between the two: partly due to their backstories--particularly the Captain's. The Captain doesn't seem to have any living relatives in the books (and in the movie, it's stated outright); though his mother was mentioned, based on his reaction, it could be that she had since passed away. His only friend, Allan, has since betrayed him. Truly alone, it makes sense that he ends up establishing a familial relationship with Tintin, once he realizes that he is true blue.
To me, it looks as though that the Captain's friends
are his family, to replace the family he may have had at one point, but lost (and that would also explain why he's so incredibly protective of them--he doesn't want to lose them, too). He's found an eccentric brother in Calculus and a sort of surrogate son in Tintin--someone he can be proud of; one specific panel that really drives the latter is in
Destination Moon, when they're about to test the prototype rocket some time after Tintin had been attacked by spies. The Captain comes in with Tintin (who still has his head bandaged), and has his hands on his shoulders as he announces to Baxter that Tintin is here in a way that translates, to me, as "
See? See what my boy is doing? He's not letting some thundering typhoon of a spy's attack keep him down!" That panel really gave me a "proud dad" vibe from him!
Tintin, on the other hand, is a lot harder to read, but it is clear that he thinks very highly of the Captain. I also noticed that he never addresses the Captain by name. To me, that's a mark of respect--we don't address our elders by name, after all, so it could be interpreted as his way of recognizing that the Captain has that authority of an older family member.
Of course, this is all up to personal interpretation; one of the charms of the series is that there is so much to interpret in whatever way we like.