That is pretty concerning. He may have gotten into rat poison. I would call your sister and ask if she knows of any rat poison at her house.
It can also be caused by what is called " hemorrhagic gastroenteritis." It causes sudden onset of severe rectal bleeding except there is usually diarrhea as well.
He may have eaten something like a stick, or a bone, and as it has passed through his gastrointestinal tract, it has pierced his colon and now there is bleeding.
All of these things are very serious and life threatening.
You may not like this but you can not stop rectal, or colon bleeding, at home. You need to get him to an emergency vet right away. Sorry : (
If he has gotten into rat poison, they will check his ability to clot blood and give him vitamin K injections which can help him clot blood. The way the rat poison works is it prevents the rats from being able to clot blood and they bleed out.
If he has hemorrhagic gastroenteritis, he will be given i.v. fluids and i.v. antibiotics.
If he has a foreign body causing rectal bleeding he will need surgery to stop it.
I wish there was something you could do at home but there really is not.
I hope this helps.