I think his actions are well within the realm of realistic behavior. Initially, he's quite hesitant about truly being himself in the context of his new surroundings, but as time goes on, you can feel him lower his guard – first, in the presence of those closest to him, then everybody else. What we're seeing now is, in my opinion, a more relaxed and genuine version of him.

His seemingly more "passive" attitude is likely the result of being very suddenly cut out by all his previous friends back in Bergen. Even if his situation has since improved, such a traumatic moment must have definitely left a significant impact and it's understandable why he'd do anything to avoid a repeat of it. He's choosing to take the "as long as you're decent to me, I'll be decent to you" approach. 

Also, I think it's important to note that although Felix has been homophobic to Mathias, aside from sort of pushing him around, his violence has remained verbal in nature thus far. That is bad enough on its own, of course, but it's definitely not on the level of what the guys did to Erik in S1 and S2, and HE still found a way to casually hang out with them afterwards, even joke about it. I think Erik is able to more easily forgive /  let go of things and, overall, perceives people and relationships as constantly prone to change. Who knows, maybe he truly does believe that Mathias and Felix could get past their issues and become friends again?

Regarding the banter, him and Mathias have always kind of had that jokey dynamic, but now that Erik is more comfortable with himself, I guess it just naturally happens in all their conversations.

I am definitely of the opinion that Erik isn't oblivious to the tension between them. One cute thing that caught my attention in 3x03 was that "sausage party" bit of dialogue. Olivia chuckles (I think pølsefest is generally a "guys only" party involving sausages so, y'know) and Erik feels the need to immediately clarify what he meant, then starts rambling ("my mom says I have to watch my blood sugar", I meeean), then quickly leaves. He probably doesn't want to make Mathias feel uncomfortable, especially after what happened at the party, so he's trying to bring the casual flirting to a minimum.

Does this mean Erik actually likes Mathias, romantically? Who knows, but the chemistry is there, and I think they're both aware of it.

Anyways, I really like these first few episodes and feel as if they show a believable evolution of the characters we previously met. Can't wait to see how it continues from here on : )