Squeeze with your legs right before the jumps and lift reins
If he’s doing that a lot on the flat it could be the bit but I would also try to lift him up while riding on the flat :) and then start jumping a couple small jumps and see it’ll take practice !
Is this a recent issue? Have you changed the bit recently? And how many strides out? By chucking his head down do you mean goes behind the vertical?
Horses do this for many reasons- avoiding contact is a large one, could be a bit issue or pain. It could also easily be a result of not enough leg to hand before the fence. It can also be because they’re bored and they don’t feel the need to watch the fence. First thing I’d do is when your horse first Puts his head down, apply leg to hand and see if that gets his attention. I’d also recommend carrying your hands a little higher. Even a short sharp tug the second he puts his head down to be like ‘hey don’t do that’. But of course it isn’t supposed to be strong, but just enough to get attention. Most importantly, add leg first and see what his first reaction to that is then work from there. I’d also try changing up the jumping exercise, like doing a vertical in a circle, or figure 8’s. Once horses realise they need to keep focused they tend to lift their heads again. I hope this helped a little!
Unlike Nora, I wouldn't recommend to stop the horse in front of the jump o to turn away from the jump, because that seems confusing for the horse. To me, the solution would be first to work on his dressage skills and on your position, to make sure that you are sitting backwards enough, and also make sure that he has his weight on his rear limbs.
Before the jump stop him when he puts his head down and then turn him away from the jump and trot away