Not that Billy isn't happy for them. No.
It's just that something inside of him feels as if it's breaking when he sees them cuddling, watches them kissing, thinks of them fucking.
Not that Billy wants to change places with Elijah. No.
It's just that he wants Dom's lips to be pressed on his, wants Dom's hands to slide down his body, wants to hear Dom cry out his name when coming.
That's all.