What?! Did we even watch the same movie?
It is definitely about the character of Jake. The movie is entirely from his point of view.
He is the films protagonist.
I might agree that there could be a better title, but to say the film isn't about the first born son is just silly

I disagree; it is Jake's story. He's the one who first cops on to how Sam is bad news, sees the drug taking, finds the drugs, etc The key events the viewer is shown are seen from his perspective.
"Your mother puts license plates in your underwear? How do you sit?!"

Indeed he did. Sometimes the firstborn has to step up to the plate when the parent or parents are lacking, and that was certainly the case here.

It's all about the first born child.How he grew up into an adult really quick and how he became the man of the house protecting his family.

I do see your point and to elaborate
upon it further let me point out a few things.

1. Jake was the man of the house for the two years after his parents split up/divorced and the Firstborn title relays that by having the majority the story's taking place with Jake in the center figuring things out.
2. The title I mentioned 'Moving In' was only the working title while filming. The book was known as Firstborn as well.
3. While the word firstborn may refer to the child who is born first. It is the title in life that brings that word to a higher meaning. Think about how in traditional era's how the firstborn would have been seen as the protector when the father figure is absent?
   That's all.
   The Metal Man From Titan
   "Till All Are One!"

"Moving In" would have been more appropriate. It's about a man moving init's not a story of the firstborn child. He's just a character.

It was originally titled as 'Moving In'. The title change may have been a last minute change when few titles were available.
The Metal Man From Titan
"Till All Are One!"

Because Jake is the first born son that must step up and rescue his family from Sam.