Pooping anywhere but the toilet

My nephew is almost 4 and hes been potty trained for almost a year now. he did so well in the beginning but over the last few months he will not poop in the toilet. he likes to poop in the hallway by the stairs and in the kitchen sink. yes, he climbs on the counter and poops in the sink the second anyone takes their eyes off of him. my sister has tried everything under the sun to show him he has to poop in the toilet and nowhere else. what else can she try? I’ve run out of ideas to help her out, too.