Tough Potty Training Situation

Hello,

My mom and I adopted two small (> 15 lbs and > 7 lbs) adult, male dogs that were previously rescued from a terrible hording situation, there were over 65 animals in one house. They got fixed when the shelter aquired them. Naturally, they are fairly anxious but fiercely trusting of me and my mom. Our problem lies in their house breaking. We think that before we got the boys, they have never been outside before. It's been well over 6 months and they do well outside when we are with them and they do very well on their leashes. The smallest one is very eager to potty outside and go right back to the comfort of my bed, which I am very happy with. However, the larger one will not potty outside at all, and will intentionally hold it until we get inside, even if I wait with him in one spot outside for over 45 minutes. He will sit at my feet and look off into the distance for the duration that we are out there. I take them both every 2 hours, right after they eat, and if I notice any ques. I'm desperate to get them trained by the end of the month so we can take them on a weekend vacation with us. We put them in a kennel previously for 4 days the last time we took a trip, and they didn't fair very well and has GI upset for a week after. Please, if you have any suggestions, I would be more than willing to give them a try!