My suggestion is to buy the litter tray as soon as possible before you move and start to get him used to it. Buy a good quality litter (Worlds Best Litter is expensive but very good) that he will be happy to use, I tried cheaper ones with our cats and they ignored it. You could also spread some litter around on the area outside that he mostly uses, to get him used to the smell and feel of it, then it will not seem so strange. I introduced a litter tray to my cats a few years ago, they were about 13 years old at that point and they were very happy to use it, especially in cold wet weather.
Best of luck, Sue.