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Two parts to the question. We have to decide whether he simply "likes" to go in the garage instead, or if he truly does not like going in the grass. This could be possibly addressed by retraining him like a puppy. Meaning, you'll need to repeatedly take him out to the grass, and when he goes, you need to reward him over and over. It needs to become a routine for him to repeatedly urinate on the grass. The reward must be given immediately while he is still on the grass, so he can develop a positive coorelation with where he is going to the bathroom. He may simply prefer not to go in grass. Even at his age, he can be retrained. Dr. Andy
I should add, that there can sometimes be issues with the grass itself. If it is uneven, long grass, or even rocks in the grass, it may be uncomfortable for him. Just another thing to consider.