Kellen was exhausted.
He was never going to tell Rhys that, but there might have been a chance that he had been right. Kellen felt he might not have felt so exhausted if he wasn't injured, and if that was the case, he really wasn't going to tell Rhys that.
Not only would he feel bad, he'd probably get smug. Kellen wasn't going to allow that to happen.
So, he did his best to keep up.
At some point, he didn't really care if he got off. It was clear to him that while he was having a good time, and enjoying his time with Rhys, he'd cum enough. This time wasn't really for him anymore, and he didn't really want to anymore.
Rhys, however, needed this.