it maybe that you are keeping him too clean' all animals need a certain amount of natural dirt to keep them going or it maybe that he is doing it because he's knows that it annoys you' dogs are like kids' if they know something is annoying you they will just keep doing it' try not to let him see that it is bothering you and see what happens'
[ps i know how you feel as we have two off them]
that's the best thing to do' if he know's that he is to stop outside as he's dirty he might think before doing it again'
my dogs are so funny sometimes' if they know they have done something wrong' they both snuggle up together and try hide!!!!!!