5 yr old poops in potty when promised reward

Hi,

My soon to be 5 yr old grandson is pooping in his pants. He was promised a sleepover at our house if he went a week without going in his pants. He succeeded and spent the night and subsequent day at our house, swimming in the pool, going out to dinner, taken shopping, etc. He was taken back home on Sunday. He pooped in his pants Monday and Tuesday. I think it is for attention but my wife thinks there is an underlying cause. Any ideas?

Thanks.... pooplexed in Florida