You know how cats never forget something bad! I have no idea how you can get him to try it again. Does he like catnip? Maybe if you sprinkle some on the top of the ramp so he can see that its stable now and won't fall. If he likes nip then maybe he will go up after it. Of course, he is a cat and cats never do what you want them too!

Thats so sweet that you got him a ramp so he can still snuggle in the bed. He's a lucky BIG boy!