Yeah, a 3 point system isn't quite granular enough for most long term games IME. Also, if you're playing the Alignment track in front of the screen, a.k.a. tracked on your character log, you might also ask about actions that shift the alignment in general.
Rereading the dialogue you posted, it sounds like he's calling any improvised action a chaotic one, which isn't how I've seen this ruled by anyone before. Players pretty much always decide their PC's actions. Chaos in our game means entropy, so it's a complexity dissolving action rather than a creative one. Killing a chaotic creature is a chaotic act, but when done to protect Order it's typically considered an ordered act as the two forces are in conflict. Killing and destroying for the sheer pleasure of killing and destroying would be chaotic. Deciding to talk to an old guy you're not quite sure is a powerful force on one side or the other, with you being in between (TN), is hardly squaring off and attempting to destroy him. If you were, then I'd say the PC was siding with chaos against a lawful silver dragon. Talking to him even though he isn't your alignment? Not quite. You could be trying to convert him after all. Deciding to take an action the rest of the party won't being joining you in? Going it alone in other words? That's pretty much the definition of neutral in my book. That's a point towards center if anything.
Like I said, if you are playing this all in front of the screen, you may want to get a better understanding of upon what measure he is ruling a 1 point shift in alignment. Or a 2 point, or 3, etc.
EDIT: P1 as a LG character may or may not know the old man is a silver dragon. The DM can't really adjudicate upon what he thinks the player knows. He can only judge the actions they attempt. If he leaves, he continues traveling. Hardling non-lawful. Even if he could know the player knew the old man was an ally (L), the DM still couldn't know you were going to attack him. You character is openly (on the PC log) TN. So not an antagonist. Losing a lawful alignment almost always comes up when dealing with chaos creatures like demons or devils. Or when engaging in group infighting. Not arguments mind you, but taking the sword out and attempting to thwack the lawful priest when you're lawful too. It's called losing paladin status. An chaos priest? Thwack away, but if you could turn him away from chaos all the better for your side, no? Think of Star Wars with Vader tempting Luke, only in reverse.
