I agree!
He should demand a retraction and an apology from Biden.
Biden is currently a private citizen. He most certainly could be sued for libel.
Even if Biden was an elected official, he could still be sued for defamation. Defamation is not a two-way road. A pubic official can almost never successfully a private citizen. Just the opposite when a private citizen sues an elected official unless the public official makes a defamatory state during the performance of his legislative duties. speaking to the media is not a legislative duty.