Buddy

He is my 100% outdoor cat (well, he sometimes sleeps inside). After my Wawa (alpha male) passed away, Buddy has been coming in more often. He still does his toilet outside but this morning I saw him hanging around the litter boxes and then resting on the litter of one of them. Don't worry, the boxes were clean and still unused.
Buddy was never really accepted by Wawa and he is sometimes being teased by the other males. But Buddy is a tough guy so the other boys do not really pick on him often. Is this his way of trying to "smell" like the others so they will leave him alone?

I'd try using vanilla on all the kitties. Then they all smell the same. I *think* you place a bit at the base of their tail & the base of their neck. I can't remember right now - someone please confirm or correct me.

It sounds like now Wawa has sadly passed, your Buddy is feeling more confident about coming in. Now the ring leader of the other cats is no longer a concern to him, he might well be trying to assert himself into the group. Especially now that the others, minus the ringleader would not pick on him. If Buddy is a 'tough guy', maybe he is trying to fill the leader gap or at least be accepted into the group. The litter is interesting too, maybe he is tryng to get the scents or that he is trying to impart his.
I'm not sure about the vanilla scent but but it sounds like a good idea to mark them all with one scent.

The first thing I think of ,when a cat is hanging around the litter box, is an UTI or an urinary obstruction (crystals), both need veterinary care. Keep an eye on Buddy and make sure he is not straining to pee or peeing little drops or urine with blood.

Also, make sure he isn't claiming the litter box as his and preventing the other cats from using it. As long as the other cats have access to other boxes, if he claims this one it shouldn't be a problem. Unless, one of the other cats will only use this box, that could be a problem. I would keep an eye on this, too.

A new cat will add its scent to the group scent by using the litter box. (And by rubbing the scent glands under its chin on furniture, doorways, etc.) That's one reason why the established cat(s) will sometime hassle the newcomer and prevent it from using the litter box. It doesn't smell 'right'. Over time, the new cat's scent becomes part of the group scent and the newcomer is accepted into the group. Because Buddy has been the 'outside' cat he is not really part of the group yet.

Please, watch him closely for an UTI or straining to pee. That's a life threatening problem and will need vet care to fix.

Thank you very much for your great replies.