Walter Jr. is (as you would imagine from the name) Walt's son. He's a good kid. He loves his parents (even as he's exasperated by their utter failure to understand him), he's trying hard to navigate the choppy waters of high school life, he doesn't get into trouble, he sets up a Website to generate money to help his cancer-stricken dad (which generates a LOT of cash, thanks to Saul's Not-Quite-Legal Laundry) and overall, really - he's a good kid. (Also a kid who seems to eat a LOT of breakfasts! Ever notice that? It's like he's hollow or something. Then again - teenage boy.)
Walt Jr. also has cerebral palsy. This condition can manifest in a wide spectrum of ways - in Junior's case, his speech is a bit slowed (his mind is just fine, thankyouverymuch), and he walks with crutches, which has made learning to drive a little more difficult as it takes him both feet to manage the accelerator and brake when using only one foot is the standard method. [Worthy sidenote: It's worth noting that RJ Mitte, who plays Walt Jr., also has cerebral palsy, albeit a less severe case than Jr. Mitte has used the visibility the role has brought him to both publicize CP and to champion the cause of actors with disabilities working in the industry.]
When Walt claims to be doing "all this" for the family, he's including Junior in that statement. Walt Jr. (or "Flynn," a name he's trying out as a way to assert some independence from his befuddled parents) has had to develop a thick skin over the years, for children are not only the epitome of Pollyanna innocence, they are also the beasts of Lord of the Flies. Teenagers in particular are quick to pounce on anybody who doesn't squarely fit into their template of perceived cool. We see this early on in Breaking Bad when the Whites are shopping with Junior for school clothes. Due to the financial situation (which includes baby Holly on the way at this point), they're shopping at a secondhand store where a gaggle of insensitive clods mock Junior's speech and mobility issues. In just about the most satisfying scene in the first season, Walt thrashes the bejeezus out of the ringleader of this pack of peckerwoods in defense of his son. Walt Jr. is (of course) embarrassed by this, but as a viewer, I find it extremely satisfying, although I don't recommend it in real life.*
It's Walt Jr. who takes his dad to task for his reluctance to get treatment for the lung cancer, reminding Walt that a little chemo is nothing compared with what Walt Jr. goes through every single day. It's Walt Jr. who lashes out at Skyler when Walt moves out of the family home - a boy needs his dad, something Skyler admits but, with what she knows, she's not sure that he needs this dad.
Walt Jr. knows nothing - repeat, nothing - about what Walt's been up to. He's been sold the "lucky at gambling" story and can't fathom that his dad would lie to him.
Season 5 will get interesting.
By the way - there are only 3 more Walter White Wednesdays before Season 5 kicks off on Sunday, July 15! Beginning that Wednesday (July 18), WWW will focus on the "new stuff!"
*Instead, take out your house key and drag it firmly through the paint of the kid's car instead. I joke, I joke!