Does this happens only while hacking? Or also while riding in the arena?
My horse sometimes does it while hacking, he likes to have his head down, to smell the grass, etc. It's ok while hacking to let him do that, so he can also extend his neck, but it shouldn't be an habbit while riding, specially if you're working him in the arena. First of all you should look for a phlysical reason and see if he's 100% ok. For example, it could be a problem with his teeth, so you should have his teeth checked. It could also be an equipment issue, so check your saddle, ask yourself and other people who know him, like friends, coach, if you're riding with the right bridle, etc. Then, if everything is ok, it's all about you, him and both of you. This is a bad habbit, and you shouldn't let him do that. First of all when you realise that he's about to do it ride with your hands a little more stronger and kick him with your legs, in a good way, of course, and say ''no''. I guess that this happens to you more often in walk than in the other gaits, right? If so, it's because when you're riding in trot or canter, your horse is more concentrated in what he's doing. And when he's walking he's more distracted and pays more attention to other things instead of paying attention to you. What you can do to try to catch is attention and get him more focused on his work during the walk is to work in a lot of circles, change directions (if you're riding in the arena, of course). If this is happening when you go for hack, you should do what dressage riders usally do while walking which is kicking the horse with your legs, but first the right leg and then left one, then right, then left, right, left, etc. (well, not kicking him, it's like a nice way to ask him to move forward instead of just sitting on him and that's it.) Always try to get your horse in the bit, even while riding on a hack, this will keep him busy too, and you can also ask him to do some leg yield and shoulder-in during the hack ;) And always try to teach him that that habbit isn't good by saying ''no'' and kick him, I hope this helps and good luck ;)