It is going to take some effort and understanding from your side to make this happen. I have a two stage plan.
1. Tell him that the only way to properly and safely support himself (this is all about safety as you know), is to wrap his arms around your bust. It might be helpful if you select your riding gear carefully to maximize the safety effect. A leather bustier is well know for its ability to enhance safety. It is clearly superior to tassels. Maximizing safety in this case is all about ensuring that he holds on securely to your bust.
2, Once you have coerced him to hop on back, try wiggling your butt against him in a playful manner. If he mistakes that as an effort to push him off the back, remind him that he needs to hold on to your bust securely and that you were simply testing his holding skills. Again...this is all about safety.
Oh...and try to keep your mind on the road...safety is paramount.