The fired Two and a Half Men star -- whose character is reportedly killed off in the season premiere -- is currently paying Mueller $55,000 per month in child support. (The two are parents to 2-year-old twins Bob and Max.)
Sheen and Mueller first split in late 2009, but their divorce wasn't officially granted until this past May. A source tells Us the parents have been "getting counseling and therapy [with] the boys" and Mueller, who has struggled with substance addiction, is "actually responding to treatment."
"She's totally gung-ho," the source adds. "He just wants his kids to be happy, and if that means they need to be a family, then he is up for it... The kids have suffered a lot, and he feels so guilty lately."
Sheen -- who also has two daughters with ex-wife Denise Richards and a 26-year-old daughter from a previous relationship -- "hates that he's made [his kids] so unhappy," according to a source. "No one believes anything he says, but the truth is he's been clean since February."