Finding out the reason why
There can be many different reasons for your horse to toss his head when ridden. Here's a great article with comments and feedback from a horse chiropractor, an equitation science consultant, and an equine dental vet offering your their hints to help you resolve the issue.
(This is not in relation to head shaking syndrome, which requires more in-depth analysis with your vet)
The article is from Horses and People magazine, and the link can be read here: