Last I heard he was actually polling higher than DeSantis is among republicans. He's a Kennedy so that's going to get him so following alone. RFK Jr has actually done some incredible environmental law work that has earned him some fans. Anti-vax rhetoric was originally a hippie left-wing movement so he's got that going for him.
Personally, I think its just that people see "Jr." and assume he's a younger candidate than Biden. He's 70.
Also, the only thing a lot of people know about him is his name, so they probably just assume he has pretty much the same politics as his dad and uncle. As the election gets closer and people start doing more research into him, I'm almost positive his poll numbers will drop.