Hi,

Try and go back to how you would do with a puppy. Pups generally need to eliminate after sleep, feeding and play. Once your dog has done any of these take him out for a walk and reward him lots once he's done well. Doing this you may be able to get him into a routine so that toiletting in the garden is reduced. It can be quite a lengthy process to re-train set ways but it does happen with perseverence!