Housebreaking 3yr old Shih Tzu

Question
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Followup To
Question -
The question is - how to stop a previously housebroken shih tzu 3 yr. male from going in the house every time he is alone.

Here's some background:
Have a Shih Tzu male, 3 yrs old since he was a pup. Housebreaking wasn't a problem all these years, he went outside fine.
Recently however he has developed a habbit of going inside the house, when we leave him at home alone even for 1/2 hour or more. We've had a baby in the house now which is 1.5 yrs old but we make sure both of them gets attention, etc.
Our dog however seems to be upset or something ever time we leave him alone in the house. He goes in the house almost every single time he's alone. This is getting too frustrating, especially with regards to cleanliness, etc.
Any help would be greatly appreciated.
Answer -
Does he go in the same place/places or is it totally random - if its the same places its probably a dominance/marking problem.  You need to work with the dog to establish a relationship where he knows definitively that he is bottom of the pack (this will hav changed with the arrival and growth of the baby) and he needs to be castrated (if not already)  
Crating him when you leave may be the answer you are looking for.  A dog will not go to the toilet in its crate provided it is not too big.  Dogs usually take very well to crating.
Hope this helps
Sarah
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Many thanks for the quick reply. A follow up...
He doesn't go in the same place, instead it's anywhere we leave him. How would castrating help? He's not at the moment mainly because he's held up an excellent rep of being very very docile, almost timid, never aggressive,etc. Thought castrating would "slow" him down even more. But regardless, this new habbit of going in the house when we leave him alone is really becomming a hassle and developing hatred towards this cute dog. He does try hard to come out with us every single time we leave the hosue but of-course we can't entertain that.
Any more thoughts from your side would be greatly appreciated.
thanks again,
Raj.

Answer
Castrating helps because it prevents him marking his territory and can also help in cases of other inappropriate urination by decreasing stress levels.  Castration generally makes for a more well-balanced dog, and of course prevents many serious health problems such as prostate cancer which has an extremely high incidence in dogs
I would reinforce my suggestion of crating.  This will hopefully discourage his habit of urinating and if not it at least means you can put newspaper down in the crate so it is easy to clean up when you get home and you can confine him to an area with a solid floor - such as the lino in the bathroom etc.  
Sarah