if you have trees, you could use the tree to tie the rider to to assist with him/her staying on the horse
or use fishing line as "guy wires" to keep him seated ... might take a lot of fishing line guy lines, but theoretically it should work ... you'll definitely need 2-3 people to help ... you can use something like tent pegs in the ground to tie down to ... if you want you could use "turn buckles" to tension the fishing line/string
probably, ideally, a combination of the 2 might yield the best results
amk
or use fishing line as "guy wires" to keep him seated ... might take a lot of fishing line guy lines, but theoretically it should work ... you'll definitely need 2-3 people to help ... you can use something like tent pegs in the ground to tie down to ... if you want you could use "turn buckles" to tension the fishing line/string
probably, ideally, a combination of the 2 might yield the best results
amk