You could try only letting him out in the garden when you are there yourself and calling him back every time he goes near the fence.
He is unlikely to take the hint, however.
Six foor chain link fencing all round your garden might do the trick, nothing else will.
Have you considered making a covered run, like a hen run but roofed in - big enough for him to play in? And training him to walk on a lead?
As he is only nine months old he can learn to walk on the lead and accustom himself to a cat run.
I personally don't care for the cat run idea, but it is better than him being run over, if you cannot afford to fence the entire garden with a tall fence.