You've not given sufficient information to enable any qualitative response.

What's he doing/not doing by way of evasion?

Have you got a short video clip or even stills showing him under saddle with you on board?

Do you have a riding instructor? What do they say?

In the meantime you might find something useful in the first post in this thread:

http://fhotd64476.yuku.com/topic/69211/Training-Horses-101#.ULzZt4ZKTII