Dressage means training and is useful in ANY discipline.
1. If the horse throws his head with contact the first thing you should do is look in his mouth. At five he should no longer have any caps (baby teeth) coming out, and his wolf teeth should have been removed. Also he should have his teeth floated every 6-12 months. At this young age do NOT go longer than 12 months.
2. The next thing is looking at his bit. With youngsters GENERALLY you go with a fatter bit. The exception is if the horse has a small mouth (low pallette) where a fat bit would irritate his mouth. Make certain the bit is not pinching the corners of his mouth (loose ring snaffles tend to do that hence why I prefer D ring snaffles). Also be sure the bit is long enough to clear the sides of his mouth without sticking out too far (1/4 " on either side is the preferred amount).
3. Now it's up to you as a rider. When you ride you have a "discussion" with the horse. Follow this link for a good article on hands. The Art of Classical Riding--The Rider's Hands
I like to think of the elbows as a spring for give and take as necessary. (The upper arms hang straight down and elbows sit on the waist, the forearms should be in line with the reins directly to the horses mouth). When you trot the horses head comes up and down (a little) - the idea here is your elbows come forward and back (a little) in response to that movement so that the horse is not getting it's mouth jerked on and the rider can maintain a steady contact with the horses mouth. This same movement is applicable to the walk and canter - but easiest to feel at the trot.
Once you can get and keep a conversation going I bet (if you resolved #1 and #2 above) he no longer throws his head up and down.