Marcy discovered she was pregnant with Stuart while dating Stu Beggs. At first, she told Stu the baby was his, but revealed the truth in ...And Justice for All, revealing that Charlie was the actual father.
Stu was born two and a half years before the beginning of Season 5, and arrived with Charlie at the airport to pick up Hank Moody. Hank sees Stuart as an honorary nephew, already seeing Charlie as his brother. Stu is still incapable of speaking, even at two and a half years old, with Marcy and Stu saying it's completely normal, which Hank disagrees with.
While peeking at Marcy and Stu having oral sex, Charlie quickly discovers Stuart watching.
Now that Marcy had divorce Stu, Stuart has been living at Karen's house. After surviving a flight aboard Atticus Fetch' airplane, Charlie spends the day with his son.
After his parents re-marry, Stuart is now living in Charlie's home.