About a 15minute drive apart
IMO, it's not the distance but not feeling a sense of being 'home' because he's feeling unsettled for some reason. It may well be something that has an easy solution but a 9 year old might not know there's an easy solution!
I think it's important to get to the bottom of the issues as it depends what he values and what he associates with 'home' and security as to why he's feeling unsettled.
I agree with PPs that duplicating all his stuff might help, and making sure he's at the right house at on the right days for the stuff he wants to do (so he doesn't miss out on movie night at mum's or a neighbour kid visiting their dad on certain weekends near his dad's, for example).
Is he in a set routine where it's the same days each week or does he feel like he never knows where he's going to be? If he's feeling all over the place, a calendar might help so he knows where he's going to be when.
Has it caused issues with arranging stuff with friends? That would be a huge deal to a child.
My DC used to get really upset with not being able to bring gifts, clothes or toys etc home from their dad's (he's an arse so different issue) - sometimes it can be a pain to pack everything they want to take (especially when he's coming back in a few days), but is he able to move his stuff freely between his homes?
I think the key is to work out what's important to him and address those issues - it might be something completely random that's an easy fix so I wouldn't get carried away with thinking about changing the arrangements until you've got to the bottom of it.