I hope I'm not irritating
I agree that he's the tallest in the room. I don't have any doubts about that. My doubts are about whether he is the tallest of these two boys or the taller of these two boys. I hope you understand me. I think of it this way: why say he's the tallest when it's enough to say that he's the taller (if he's taller than the other and there are two of them, then of course he's also the tallest). He is the tallest of them in a mathematical sense. But would you say that? I wouldn't neither in Polish nor in English.
PS: To make my point clearer:
Tom is the tallest in the room.
This sentence must be as it is. *"Tom is the taller in the room" doesn't make any sense.
Tom is the taller of these two boys.
This is the sentence that I'm talking about.