That's the million dollar question. lol
At night, it's the hardest time to pee pad train unless you crate and get up every couple of hours. If you need to do the xpen thing, just reinforce a lot during the day when he uses his pee pad..praise praise praise and even a treat (I never treat for pottying but many do so it's your call). The only other option is to get one of those puppy apartments. This is actually a good idea. It has a divided section for sleeping and another section for pee pad. They, instinctively, don't want to pee and poo. I would skip the xpen and go with the puppy apartment. It is a little costlier but you have time to save and get it between now and next month
