What about gradually moving the tray towards the back door (if it's not there already), maybe putting a bit of garden soil in the tray with the litter and most important, having a very well dug patch of soil as close as possible to the back door that he can really easily scrabble into (and maybe a bit of cat litter on top so he understands what it's for). If you make the garden convenient and easy to use he's more likely to be willing to toilet there.