Hallo and welcome! Have you found a name for your bun?
Litter training needs patience and time. Do read the relevant articles in our Library section in regard to litter training and litter boxes.
Since your bun just came into the house, this is a completely strange environment for him. So it's only normal for him to pee and poop everywhere to mark his territory. If he's young in age and not neutered, this adds up tothis behaviour. So, first of all, give him time and let him relax in his new surroundings. Don't press him too much before he starts feeling at home.
Try putting some of his poos in the litter box, so that the smell leads him there. Try putting hay in it, most buns love to eat and poop at the same time. This will also make him eat even more hay, which is extremely beneficial to his health. Whenever he goes to the right place, i.e. in the litter box, give him a small, healthy treat, to make him link in his mind that going in the litter box is followed by the pleasure of eating his favourite treat. If he mistakenly pees out of the litter box, never shout or reprimand him. Just take him to the litter box and make him see this is the correct place to do his business. Little by little he's eventually learn. Don't be discourage by relapses, repetition is the mother of knowledge. Try again and again and be patient with him, he'll do it right with time.
Marietta