Hi!
"So apparently Tanaka is just a Phantomhive house Stewart right?"
Jup! Just a head butler turned house steward :)
(Though what he was before he started working for the Phantomhives is a mystery!)
1 - No, Tanaka is not related to the Phantomhives! They just call him "Gramps" out of affection.
2 - I doubt this will ever be explained as it's just a silly gag. Don't think too much into it. (But then, you can never know what is "just a gag" and what isn't in Kuroshitsuji... So, don't think too much into it - for now.)
"Or are they going to give us his back story in the upcoming chapters?"
There's no doubt that we will get his backstory sometime. After all, we are (apparently) getting backstories for all servants, and Tanaka's backstory is the most connected to the Phantomhives. He has been working for them since at least Vincent's childhood (considering that Japan was isolated for centuries until 1853, I don't think he got to England any earlier than that) and was, most likely, employed by mystery granny Cloudia Phantomhive herself. Also, he's a Japanese man with exceptional katana skills in 19th century England - he surely must have been "someone special" before he became a butler. Maybe an assassin like Mey-Rin? I don't know.
Well, anyway, considering where we are right now in the manga, I don't think his backstory will be "in the upcoming chapters". I think we'll have to wait the longest for his backstory. I guess Tanaka's backstory may come up around the time we get Cloudia and Undertaker's? Because Tanaka's backstory does involve Cloudia to some degree.