I have one of the Kentucky Horsewear stall pads. It’s for a horse who paws at stall gates. It is very light and has grommets all the way around. But it is also very tough. They are meant to be portable to use in trailers and at shows also. I wonder if you could rig something like that to hang onto the back so he would kick that before making contact with the ramp. Much easier to move than a normal stall mat. And vinyl covered so not hard to clean if pooped on.
It does depend somewhat on why he is kicking. For the pawer, he wants attention and likes the noise. The pad makes it much less satisfying to bang on.
My other horse doesn’t kick or bang on stuff much but paws with the front legs in the trailer a lot. He usually stops once we get going, especially if he has a buddy. He is a little claustrophobic about trailers. Having some windows open helps him a lot with this behavior. Which I don’t do much where we live because it’s often highway driving to shows. But we’ve done some shorter trips where I’ve experimented with it and noticed a big difference in his anxiety level. Don’t know that I’d want to do that with the back windows for one that’s already testing the ramp. But maybe a thought for the around the parking lot type of driving experiments.