Seeking back float help

We started swimming lessons later than I wanted, around 3rd birthday, and we’ve been doing it for a few months now. But he refuses to do a back float. He’s still in parented classes. He will splash around happily with floaties on and will put most of his face in the water of his own volition. However he just bends whenever it’s time to do back float. He can’t pass to the next level until he does the back float. He’s definitely sensitive/orchid child. Wondering if anyone has had any success with their kids. Thanks!