Your Wheaton probably is having a problem because he came from a pet shop. That's probably what's happened here. If someone were willing to get up about 3 am and take the dog out, giving him an opportunity to relieve himself outside the crate, that could solve your problem. He will grow out of the need to go at night in another month or so, and if you could work with him to keep his crate clean for that amount of time, you probably woudn't have the problem any more. I know it's gruesome getting up in the middle of the night, but you can sort of do it without waking up fully once you're used to it.
Dogs are different in their toilet habits, just as are people.
If he regularly has 5-6 bowel movements, and they aren't runny or mucousy, that's probably the natural amount for him. It seems whatever I do, he reacts like it is all part of a game. Boxers are also known for being fun-loving dogs. You can try a couple of things. Only train for two minutes at a time.