What is the ruling on the prayer of a disabled person who suffers from involuntary drops of urine on his clothes due to the difficulty of changing them, and is it permissible for him to perform tayammum with unmoistened soil from the garden clay?

A Muslim must be careful to prevent urine from getting on their clothes as much as possible. If it does get on them after making an effort, then only the impure spot needs to be washed. If one is unable to do so, there is no blame upon them. Water must be used for purification before prayer. If one is unable to use all or some of it, they may resort to Tayammum (dry ablution). Tayammum is valid with pure, permissible, earth that is capable of producing plants, and this includes dry garden soil.