Understanding Resident Cat's Behavior with New Kitten?

We have a big male cat that has been neutered since he was little, named Roman. Well a few weeks ago I found a kitten in the rain and decided to keep him, he's only at the most five weeks old. I had to bottle feed him at first, but he is eating food and gaining balance haha. (His name is Percy.)

They both get along just fine, they play, sleep together, and purr and lick on each other. Roman even helped by cleaning Percy's butt every once in awhile to help him go to the bathroom.

But my question is sometimes when I decide to let them both in my room, which originally was Roman's room fyi. Roman starts to meow loudly and almost moan-meow in a way and he will grab Percy by the neck and sometimes carry him, even though he's gotten a little bigger and it's hard for Roman to carry him, but sometimes he just pins Percy down.

Anyways, it only happens every once in awhile, but I watched him do it tonight and it almost looked as though he was mounting him. Even though Percy is way too small. Sometimes it happens when Percy is too hyper and it looks like Roman gets frustrated, maybe it's his way of saying he doesn't want to play?

I'm just curious if this is a problem or is he just claiming Percy like adopting him? Or is it dominance? Sexual aggression?

Anything would be helpful.

Like I said majority of the time they get along, and I don't even know if it's a real problem because besides this, they love each other.