My dogs been unwel but getting better?

But the problem now is he's not had a pooh since the Diarrhoea stopped, he was given an anti sickness injection and an antibiotic one on Thursday , I had to take him back on Friday and he was given another 48 hour injection, i've only to feed him rice and fish until i take him back tonight along with a Pooh Sample. How do i get a pooh sample if he's not doing it.