I doubt he is injured because he is still very active, (jumping, popcorning, running, etc.) and is eating well as usual. I suspect he isn't using the ramp anymore because everything he wants and needs is in the bottom level of his hutch, so he has no motivation to go up the ramp at all. However I will still give him a brief physical check myself just to make sure. I'm in a fairly isolated location, so I can't see a vet very easily.
He is around 1 1/2 years old, the ramp isn't very steep and has a non-slip surface but lacks sides.