It helps if you think of "mudar de" as "to make a change of." So in this case it would be "he does not make a change of subject."
Can you translate "muda" as "refrain from"? Because why should the preposition "de" be used here instead of an article?
Mudar here means to change to another subject. I don't think "refrain" carries this meaning.
Also, mudar requires preposition DE:
Why we do not have here "muda do assunto" ? English translation is "the subject".
Because we do not use the definite article in this case, unless you specify what the subject is about.
He is not changing the subject?
Depending on the context, it should also be accepted.