Is it possible to convert this:
He is not so busy that he can't go for a walk now and then.
to this:
He is not such a busy man that he can't go for a walk now and then.
without changing the meaning?
I don't think it changes the meaning substantially (unless, of course "he" is a boy, and your rewrite aged him into a man).
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Good. Thanks, Barb!