Hello Lucy! I think it's normal for a young horse to wants taking off his rider. But if that takes too long maybe your saddle has inappropriate size for him. Usually is due to pain, fair or confusion.
Maybe you’re coming on too strong. Instead of incorporating devices for severe stop aids, consider a lighter bit, a softer hand, releasing cues, a looser tie-down or martingale (designed to prevent a horse from raising his head to a dangerous height – not to hold it in a certain place). Really think about how your horse responds to your requests. Removing the pressure increases understanding, so it’s no wonder intensifying pressure without release usually doesn’t work.
Good luck! :)