I figure it's worth pointing out, Sion's core is frozen heart and he isn't getting any love like the other tanks, his base stats are rather bad for a tank role. Some quick comparisons.
Sion
ARMOR 23.0 – 74.0
Yi
ARMOR 24.0 – 75.0
Nautilius
ARMOR 26.5 – 81.7
Nidalee
ARMOR 22.9 – 82.4
Sona
ARMOR 20.5 – 76.6
Sion is given poor base tank stats because he has the potential to inflate his health more than any item could by farming (with this health being increased further by cinderhulk). Whether you agree with it or not, this is the reasoning Riot has given.
And? Taking one set of stats and comparing them out of context is pointless. Sion is one of the tankiest champions in the game so apparently his low base armor values aren't getting in the way. Also he's going to massively benefit from the new items they are releasing, especially Titanic Hydra.
Sion has less base armor then Master Yi
Generally speaking light fighters, assassins, and anti-carries expected to never get defensive items have the best base defensive totals, where tanky derps, heavy fighters, and tanks trail behind them.
Also - generally speaking base stats are to close to care about. Which is also a really good design choice, because it means 25 AD is 25 AD and 100 HP is 100 HP when you grab it on different champions, causing ability ratios and synergies to be the only thing you have to bother with.
Also good because item interactions could get really awkward with massive differences in base stats. If you have a tank with 1000 HP at level 1 and 50 AD/50 DPS, and an assassin with 500 HP and 100 AD/100 DPS, it will take the tank 10 hits to kill the assassin and the assassin 10 hits to kill the tank, balanced. But then you run into this awkward scenario concerning itemization: A +500 HP item on a tank will only increase their HP by 50%, where a +50 AD item will increase their DPS by 100%. A +50 AD item will only increase the assassins damage output by 50%, but a +500 HP item will increase their defense by 100%.
If both get the tank item: the tank (1500 HP and 50 DPS) will require 20 hits to kill the (1000 HP 100 DPS) assassin, where the assassin only requires 15 to kill the tank. Alternatively, if both get the offensive item - the tank (1000 HP 100 DPS) will kill the (500 HP 150 DPS) assassin in 5 hits, where it will take the assassin 7 to kill the tank.
Looking at most base stats? They're pretty much the same and nothing to be concerned about, differing by about 4% EHP on the champions you threw up. I'm also pretty certain Riot uses base stats more to nudge a champion in small directions and essentially deal with the remainders and left-over odds and ends that would cause a 100/150/200 damage scaling ability to do 101/148/195 damage otherwise.
Then there's also champions like Jinx that use notably different base AS to get away from AS penalties when using a rocket launcher, but for the most part - look at a champion's skills rather than their bases, and you're usually safe assuming their bases are the same for the purposes of mental math.
As someone who plays Sion quite frequently, I really don't think he needs any buffs. He is one of the tankiest bastards in the game and he has meaningful damage along with heavy CC and lockdown. It is sorta common for Tanks to have shit base tank stats because they are expected to build more heavy on the tank side and you can see that the opposite is true in that Sion has incredibly high base AD, one of the highest in fact. He's in a fairly balanced state right now and the only thing that keeps you from seeing him in pro-play is possibly how prevalent Gnar is.
That metal must really help because 
has 90 armor at level 18
##Sion has less base armor then Master Yi.
Sorry, it was one letter that I couldn't help but say was wrong. 
