Don't ya know how he feels being away from his toilet?
Not sure why this is a "new" problem but when Shelby was 5 months old we took our first road trip and she would NOT pee. little puppy went 8 hours!
Now we practice strange pee pee places all the time and she loves it. We walk together every day and she loves peeing and pooping all over town, especially where other dogs have gone.
Practice.