Howe, 86, suffered a "pretty bad stroke" on Oct. 26 while at his daughters' house in Texas.
His family released a statement on Nov. 4 saying Howe was recovering at a "remarkable rate," but Mark Howe said things have changed since then.
"His health has taken a turn for the worse this past week to 10 days and we are doing what we can to help his situation the best that we can," Mark said. "But he is not doing well at all is the bottom line."
Howe was able to watch video of a tribute the Detroit Red Wings paid to him at a game earlier this month, which his family said made him noticeably emotional.
Mark Howe also recently told "The Hockey News" that his father's health is "headed in the wrong direction."
Howe played 32 professional seasons, including 25 with the Red Wings. He is the all-time leader in regular season games played and most games played with a single team.
- Molly Geary