just turn the faucet on while he's holding it. The running water will mess with him and he won't be able to hold it for long

>...my son refuses to use the potty over here at my sisters
>house. flat out refuses and he's wasting my money on pull-ups.
>at home, altho he's going through a lil regression, he'll use
>the bathroom as long as i remember to ask or bring him. here
>he will sit on the toilet and HOLD his pee. as soon as i give
>up and pull up his pants etc he pees. its infuriating but
>i've given up on trying while i'm here lol i can't deal.