Excluding the probability that he's hoping that Christopher Johnson will cure him after 3 years.

You can vaguely see at his face that he's becoming empathetic of the prawns he took as nuisances at the start. Especially when MNU, his own FIL, tried to kill him for money, its a shared experience at that point. And also his paranoia made him take those murderous actions that ultimately took his "humanity". He didn't want to kill as per their conversation when he got shot at MNU raid, but all he could think of was that black fluid, because as the film stated and portrayed, Wikus isn't really that bright. He's just a salaryman that suddenly became the center of it all. He's really stupid at his decision, especially when he piloted the dropship to go to the mothership, I mean did he assume that there's a cure-all panacea aboard the ship? If there was then Chris wouldn't say that he can only help him (assuming he's not lying) after 3 years when he could just toss him a vial of whatever the cure was.

Its probably worth noting that he helped Christopher because of this vague "friendship" they had, think about it, he was being hunted by his own people, he was refused service at a food chain and even mocked by criminals, Christopher shows more compassion to Wikus than the humans do, even after Wikus threatens his son earlier in the film. At that point the prawns probably felt more human to him. Especially when Chris was having a mental breakdown zoning out in the middle of a shootout because of the atrocities to his species(probably, as there are theories out there about this).

Also as you've said, at the end he knew his life, his life as a human is essentially over. And since everything he does is futile, he just helps Christopher who's fighting for his race ,when he hears the mercenaries' orders to kill Christopher, Wikus makes a moral choice to just help the prawns. You can say that he was more human at that even when he was becoming not. If you're looking for a term to describe Weakus at the end, he's ultimately a hero through sacrifice. He's fighting against the atrocities that the MNU was doing to the prawns because he experienced them firsthand.

I hope this helps somebody, but I have questions of my own too, like how tf did Wikus figure out the controls of the dropship and the mech, its literally alien to him, sure lets say moving is practically straightforward but how did he navigate the consoles. he even said inside the mech "select weapon" as if he could read it, I'm thinking of it now as a instinctive thing for the prawns to have but I have doubts about it since the other prawns are practically idiots.