Thanks M&M
It has been lowered already - along with a lower ramp. He was a bit stressed about that change along with his vet visit, and has just got used to it. I tend to raise the shelf to roof level (ie flush against the top bars) rather than removing them so it still gives the overhead cover. I'll gauge the timing. He gets up and down there easily at the moment but I do worry he might topple off the edge - his balance is better up there though because it's flat.
Next thing is to flatten the substrate so don't want to change too much all at once as he hates anything changing!
Yes he is doing all the normal things, running on his wheel and saucer without issue, moving around the cage fast - apart from having difficulty balancing on the loose substrate. He doesn't seem in pain - but I'll keep an eye on it and watching his movements etc.