OP how long are you asking him to hold it for. I know mine when healthy can all hold it for +6 hours but are rarely left for more than 4 hours. In my opinion every accident is a small step backwards. I would resist the space he has. I used a playpen for mine til then were very reliable about holding it and going outside only to potty. You can buy a baby proof dome for them too if needed. It worked great for me and mine. I still use it when they are sick.
Graco Sport Pack n Play Dome Tent Sun Shade GREAT | eBay
You can do something similar with the x-pen using netting or plant netting. I had to do this with mine because my littlest Roxy would escape the x-pen by jumping right over the wall. The netting stopped her.