Some dude on reddit brought up this possibility... would be the biggest holy trout moment of the entire show:

Quote Originally Posted by Some dude on reddit
Wouldn't it be ironic that we find out that [Walt Jr.] has been smoking meth with Lewis this entire season? I see it possible because we (and the characters) have been more focused on Walter and Hank situation and how that will play out, and less focused on the lack of attention being paid to Walt Jr. A great twist would be for Walt Jr. to OD on Heisenbergs product, driving the family apart causing Skylar to leave Walt (hence the flash forward to a disheveled Walt). To address the theory of Walt killing Skylar, I think Walt making his age in his breakfast is a misdirect (not that he kills/killed Skylar) but that he misses her so much, after she leaves him because of his meth being the cause of Walt Jr's death. Just a cool twist that I haven't read yet, so I figured I'd share it.
he has been suspiciously absent the past few episodes...