Every custom saddle--not just dressage saddles--needs to be checked every 6 months. I always have mine checked regularly, and several times I've needed the flocking added to, or reduced, due to the horse's muscles changing. Since your horse flings his head while going downhill--and if you DO get your saddle checked often--I would suggest asking a chiropractor or body worker to look at him.
Another piece of advice: he may be protesting the bit. You say he doesn't "collect"--do you actually mean flex at the poll and"go on the bit" when you say "collect"?--and if you're using the reins to get vertical flexion I can see why he's throwing a fit. When you're going downhill do you use the reins to keep him from rushing? Again, he may be protesting your heavy hands when you pull on him to slow him down.
To get him to give to the bit happily, use just a few OUNCES of pressure on the reins--a very subtle amount--and keep the pressure the same for as long as it takes for him to flex at the poll. He will try different things to evade the pressure--even though it is very light--so follow his head with the reins as he moves it. He'll try bending right, then bending left, then putting his head up, then backing up, but just keep the same light pressure as you follow him with your hands. Eventually he will try lowering his head and flexing at the poll: when he does, tell him "that's the right answer" by loosening the reins immediately. Just drop them. Give him some time--maybe a min?--to relax and think about what he did before you ask again with the LIGHT pressure. Once he gets the hang of flexing for a second, ask him to hold the flex for two secs. Then 5 secs. Then 15 secs. You know where I'm going with this. It may take days to get him to be happy with flexing at the poll for just one circuit around the arena, but that's what you need to do: be patient. Not only will that make him happy about going on the bit, it will make him SO light in your hands, and that's what you want: a horse who happily works with you from just the smallest aid.
ETA: Flexing at the poll is hard work. That's why you go in small increments, slowly, in increasing the time he stays flexed. Not only does he need to learn to hold the flex, he has to gain the strength he needs for flexing.