Brendan Johnson, a Sioux Falls lawyer, said in an interview with The Associated Press that his father has been responding repeatedly to directions from his mother, Barbara. But he is not yet speaking.
"It's fair to say he's been exceeding expectations up to this point," Brendan Johnson said. "All the tests and the indications now are positive."
Johnson offered few details about his father's condition and said he doesn't know what kind of tests he will undergo this week. He said he is not sure if the senator will need additional surgery.
He said he left Washington this weekend for a brief trip back to Sioux Falls but he will be back to celebrate his father's birthday next week. The senator turns 60 on Dec. 28, and he is expected to still be in the hospital.
Johnson was not sure how long his father would be hospitalized.
"At this point I really don't know what kind of time table they are setting," he said.
Johnson's spokeswoman, Julianne Fisher, said Tuesday that Johnson remains in critical but stable condition and is still sedated, though he has been conscious at times and his CT scans "continue to go well."
Johnson was diagnosed with arteriovenous malformation, a condition, often present from birth, that causes arteries and veins to grow abnormally large, become tangled and sometimes burst. He was rushed to the hospital Dec. 13 after becoming disoriented on a call with reporters.
Meanwhile, Sen. Charles Schumer, D-N.Y., said at an appearance in his home state that he has received word that his Democratic colleague is making "a very strong recovery."
"In fact, they think right now that there's a very good chance that he could get all or most all of his functions back," Schumer said.
Barbara Johnson issued her own statement through the senator's office Tuesday, saying that her husband "continues to move in the right direction."
Johnson showed some signs of recovery late last week, responding to voices, opening his eyes and moving his limbs. His long-term prognosis remains unclear.
The senator's sudden illness raised questions about the Democrats' one-vote majority in the upcoming Senate session. South Dakota's Republican governor, Mike Rounds, would appoint a replacement if Johnson's seat were vacated by his death or resignation.
A Republican appointee would create a 50-50 tie and effectively allow the GOP to retain Senate control because of Vice President Dick Cheney's tie-breaking vote.
There is ample precedent for senators to continue to hold office while incapacitated.