My pomeranian puppy has diarrhea with blood and straining to poo but tested negative for worms and parasites?

I adopted a Pomeranian puppy over a month ago; when I got him he was active, playful with very normal stool. A week ago, I notice he had a bit of diarrhea with traces of blood in his stool so I took him to the vet and had him wormed and tested for parasites and his test came back normal. However, my vet believes that he has coccidian (due to the blood in his stool) and prescribed him some antidiarrhea and internal parasites medication. I also switched his diet to rice and chicken, nothing has changed and now I notice he has a really red protruding . I took him back to the vet she ran another fecal test and parvo test (both came back negative) and she confirmed that he has rectal prolepses and had to put a suture around his bum for 3-5 days. After his minor surgery he was still eating and playing, but still straining during bowl movements. He is supposed to have his suture taken out tomorrow, but I am afraid his rectum will pop out again due to his straining. Any advice??