Shiba Inu house breaking regression

Question
We adopted a shiba puppy 11 months ago;housebreaking went fine, though he was crate trained when rescued, and went through a brief period of trying to eat his poop if he had an accident in the house. He is 13-14 months old now. We adopted a 6 yr. old eng. mastiff, also male, one month ago, and the shiba has calmed down considerably, (that was the idea) and they only have an occasional conflict about treats; the shiba accepts the limit set by the much larger dog in those situations, and the mastiff, who came to us very well trained and gentle, actually tolerates the shiba's less desirable traits such as nipping at his legs or licking his substantial lips instead of making him stop, which we were hoping he would do! The problem is this: the shiba has started going to the bathroom upstairs recently. We have been vigilant about taking him out more frequently, are making sure to give him affection in case he's jealous, but have resorted to putting up the gate in the doorway to the upstairs. He will go outside and NOT relieve himself, even to pee, then sneak upstairs shortly afterward when we arent looking (like when I go outside to get the mail and leave the gate off briefly) and make a mess. My daughter believes he is doing this deliberately and that I should reprimand him when he does this. Because of his poop-eating issue as a puppy when we first got him, I tried not to scold him when he had an accident, but praise him when he went successfuly outside. But he's been successfully house broken for a long time. My question: how do I stop this behavior? I'm so tired of stepping over gates!

Answer
Hello Michelle. Well i would really need to see the dogs to evaluate them but this is either one of two things. This issue could be because there are two males in the house, or just something that has started due to some confusion with the dog. Either way, you must not leave the dog unattended, even to go get the mail. You need to use the crate again and pretend he is not housebroken and start over. Scolding the dog will not help, but this process should not take long to fix this time if he was doing well before.
Good luck and keep me updated...
Nancy