What is the best way via chat to ask him whether he has fully
recovered from his chickenpox?
Just be friendly and express concern for your manager's health.
Just ask something like "Hi. How are you feeling? Fully recovered? I heard that you might be coming back to work later this week?"
You'll likely get a response on the order of "Thanks. My doctor says that I won't be contagious and I can go back tomorrow."
That said, just stay away from your manager for a few days when he returns. The virus is not as easily spread as most. It can be spread through coughing and sneezing, but is mostly spread by contact.
If necessary, make a joke about it when he returns. "Ugh! I never got the vaccine and I'm not getting near you!" when said with a smile and a laugh won't be offensive.