Litterbox training an older bun?

So I've had Oscar for about a year now and he's still having trouble with his litterbox. He almost never pees outside of it but does poo outside of it quite a bit. Oscar was 7 when he was adopted which would make him 8 now.
I'm wondering if his litterbox issues might be due to his older age? I'm not sure that he was ever litterbox trained at any of his previous homes (the shelter says I'm the 4th home the little man has had &#128546. He's neutered, and he lives with a spayed female who I know uses her box pretty darn well.
Any suggestions on how to help him? I want to try bonding him and his female Chloe with my other bonded pair... Just looking to try and get everyone on their best behavior before it happens

If he isn't peeing outside his box, then he is litter trained. The poos outside are fairly common -- most especially in the presence of other bunnies. After all, he has to mark his territory. It is the rare bunny that is 100% with poos.

Do you have any tips to reduce the amount of poo outside of his box? I agree 100% that bunnies will poop out of the box- but it seems he poos outside of the box about half the time in comparison to my other rabbits who all do most of their poo in the box. Or do you think that he may just be a little different and mark more?
Thanks for your help!