From what I've read, if a horse has a roaring problem it occurs on inspiration. When a horse canters, inspiration occurs during extension and exhalation occurs as the front limbs land. So from what I can see in the video, the horse's loud breathing sound occurs as the horse is landing, so during exhalation, so apparently he is not a roarer. If anyone sees it differently, please comment.
If this is the case, either the horse is out of shape (as people have noted he is not particularly well muscled up) or the person with the video camera has the "zoom-microphone" option on and it is pointed at the horse's head. Sometimes that makes things seem louder than they are in real life.
To me the horse seems decently put together and obedient. If you wanted to do higher level dressage I would have some concerns about the overall length of the horse. Some of those really long thoroughbreds have difficulty getting more than a degree of collection. He seems very coordinated for his size though, so his mind may help him go farther than I might think at first glance. To pick up the canter from a walk with his length of body would probably require more training than just basic dressage.
Sometimes I prefer a horse that is a little older and hasn't been used very hard yet because they will last longer. He still has plenty of time to learn many things before he is old.
Just fatten him up a little and add some muscle. He will be gorgeous.