I'm glad moggie's settling in so well, Evian. Re the litter, I don't think any of them will last long in rain to be honest. I'm wondering if you could construct some sort of covering to go over the tray. (You can get enclosed litter trays but they are very expensive.) Alternatively, perhaps you could mix your usual litter with some of the gravel, so it doesn't feel too weird for him
I've heard other people use cat litter to mark the edge of the garden successfully. I tried it with Basil when we moved house, but the local cats seemed to see it as a challenge and there were a few punch-ups before they sorted out their territory. I guess it all depends on the cats personalities.