Problem is, who is he going to buy an ad with? All the media are out to take him down.
Federal law requires that all commercial TV and radio stations in the USA sell him (and any legally qualified candidate for federal office) ads within 60 days of the general election. See Title 47, United States Code, Section 315. If stations refuse to sell ads, they can lose their licenses under Title 47, United States Code, Section 312(a)(7).