Is そうだったんですか　different from そうですか？

I've recently started messaging with someone from Japan, but I've been struggling to properly translate them. Up to now I've always sort of guessed using context and what I do know, but I'm really curious to know the proper translation.

His response to me telling him that I'm currently studying Japanese is " あっ、そうだったんですかｗ".
I know that そうですか is something like 'Is that so', but I can't imagine what the ttan adds to the phrase.

Is there a big difference between the two or is the second option something comparable to slang?

Thanks in advance.

だった is past tense of だ（です） and ん is colloquial version of の.
This の doesn't typically make into the translation because there's no direct English equivalent; it gives it a more natural explanatory/inquisitive feel.
So the translation is: Ah, was that so?
I'm not sure why s/he is using past tense. It's kind of like they are putting 2+2 to together like "Oh now I see" or "Oh now it makes sense."

The past tense shows that he is talking about past things, for instance, previous conversations between you make sense to him now, but the present form そうなんですか also works fine and there is not much difference in that case.