Dog issues here please help?

so i have a 3 year old golden retriever who is the most loyal and trustworthy companian you'll ever find and i left him at my family's to petsit while im out of the state this week. however, hes been there for about 12 hours and won't go potty! when he was a puppy he did that too but it was for only a day and he went right when he got home. at the family's, they live in the city. theres no grass, not even in the parks! my dog is acustomed to a green environment. so im kinda worried now! also they have him in the gargae as his living space since they are kinda neat freaks and he sheds. so if he went in there would that ruin his house breaking? would he pee in the house that is his living space when he gets back here? thanks!