Well, people probably already confirmed that he's not really Steve, but, what the heck, I wanna take a shot at justifying that it is still really him.
1. Chris asks him, "Have we met?"
Although the line probably does just say that Jake looks a lot like Wesker, another possible meaning would be that they really have met before in the Rockfort Incident (I'm pretty sure Chris and Steve met in Darkside Chronicles but not in Code: Veronica).
And heck, why did Chris ask that question? If he definitely looked like Wesker, he would've said "You look like someone I know" instead of asking him if they have met. (I mean, come on, who the hell would forget his all-time nemesis' face?)
Afterwards, Jake replies with "Well, all you jarheads look the same to me pal, I'm sorry." Okay, it's a typical answer by a badass mercenary like him, but conspiracy-wise, he might just be trying to deny that he had met Chris before out of fear or reluctance into him finding out his true identity. Or I might just be paranoid. But then again... *Jake makes a dramatic look out of the helicopter after their short convo*
2. Jake has Wesker's blood.
Wesker did say that he was going to turn Steve like him so... o.o
With him being a sort of super human, taking punches from deformed beasts such as in the trailer without even taking much of a scratch on the face, he does in fact, have Wesker's blood, which was already mixed with that special virus that Wesker had. If Wesker really did make Steve like him, the he'd be godly like him. And Jake certainly does look a bit godly with the fact that he's just using hand to hand combat with the infected. And Wesker turned Steve like him, doesn't it make him a sort of fatherly figure considering that he gave "birth" to this new Steve?
3. Age.
I don't get the whole picture of it yet, so I apologize if this part doesn't really make sense.
Okay now, we all know that Jake has got to be at least like, what, 20-30 years old? I'm pretty sure Wesker doesn't have his "unique" blood till only after his first death in RE1, and if he made a baby then he'd only be like 10~ years old or so. Unless of course his blood is already unique prior to that. But either way, I think it's pretty hard to include in the whole RE timeline. Plus, back at number 2 (Jake has Wesker's blood) we know that Jake "inherited" Wesker's powers in some way. So definitely, Wesker' would've been already infected by the special virus before he made a baby.
But hey, Steve. He was 17 in 1998 (or 1999?) during the Rockfort City Incident. Considering RE6 is ten years after the Raccoon City incident, he'd be like 27 today if he was alive. Seems to fit the age, no?
4. Red hair.
I guess it says it all. Being "biologically" related to Wesker but has red hair... That's kind of odd, don't you think? Well, unless blonde is recessive, but I'm pretty sure blonde hair is dominant in Wesker's genes. Then again, Steve has red hair, and if we take number two (Jake has Wesker's blood) into account...
5. His history.
Not much was still shown on how he actually became a mercenary in anti-government operations in the first place. I mean, Wesker lives in Raccoon City, pretty sure he's been there for a few decades in his life considering his affiliations to the RPD as a S.T.A.R.S. captain. So how did Wesker's "son" end up all the way as a mercenary in eastern Europe? Experiments and manipulation are too general to say, but otherwise, it's kind of odd how he ended up there even though Wesker lived for quite a while in Raccoon City.
It kind of makes sense that Wesker may have sent a newly-formed Steve there, or perhaps Steve was able to escape from the hands of Wesker and, after desperately looking for a job, ends up being a mercenary due to his sheer strength. It makes a lot of sense, don't you think? Steve escaping from the clutches of Wesker, wandering aimlessly till he gets to this "Edonia" and was suddenly recruited after he was found to have pretty supernatural powers. There he wished to go away from the "past" and just do some destruction with his awesome powers. Yay! But then he meets Sherry and all hell breaks lose once again.
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Alright, wall of text, yeah I know. Hope you guys take the time to read, since I need someone to disprove my crazy theories, lol.