Holding onto the Handle Attempting a Backroll Help

My son has been trying to land a backroll now for over a month. He has the rotation down, lands (most of the time) on the down side of the far wake and looks like he has it but he can't hold on to the rope.

He says the rope is getting pulled from his hands but watching him it looks like he is just letting go.

Don't know if he is scared, not pulling back hard enough or if it is really just getting pulled from him.

I don't wakeboard so I am no help to him. Others that do and have watch him say he has it, he just need to hold onto the rope.

Any suggestions or tips??

Thanks