Advice on a stubborn Boston Terrier

I am fostering a year old Boston Terrier for a few weeks and he's every bit as stubborn as the breed's reputation, and probably 40% more than that.

He's "house broken", and by that I mean that he holds his water all night and immediately goes outside. If he's in his crate or in a shallow box with a pad that we keep him in when we're home watching TV, he won't tinkle or anytime we're in eyesight for that matter. He knows he's not supposed to and won't when we can see him. He usually poops twice a day and does so outside.

So the problem is that if we leave him out of the crate and by himself, he might "mark" something.

This morning my wife left to meet friends and left him on his leash at the kitchen table. First time I came down to get coffee he was sitting in his crate in the kitchen happily chewing his Kong. Second time I came down he looked guilty as sin itself. If you've owned a Boston, you know that face. Sure enough, there was a small spray under the kitchen table. I rubbed his nose in it and locked him in the crate. Is that the wrong way??? I don't have a clue.

So my answer is whether he can be trained not to mark. I'd like to get him sorted before I place him somewhere because he's already been through a lot with his first owner dying and now us as a temp foster. I'd like his next home to be his last but if he marks his new home, I'm afraid he'll be moved again.

Any suggestions?