Sure it makes a difference. He’ll be exposed as the sordid liar that he is.
10% of people have the paternal DNA of someone other than their legal “father”.
Are they sordid liars?
Most are unaware of the situation, as the relevant act was some 9 months before they were born.
So while it may make his mother look like a sordid liar (and a slut), it wouldn’t do much to him; and it certainly wouldn’t be the grounds for any legal action.
Otherwise there are millions of birth certificates out there than need amending!