Truth time
After Blue confronted Holden he actually felt better. He felt like and Blue were really friends now because he wasn't keeping anything from him anymore. He was a little worried Blue would tell Lottie, but Blue kept his word. Holden should have expected that, Blue is nothing if not trustworthy.
But the relief didn't last long because he didn't have an excuse to keep this from Lottie anymore. He loved her and she loved him, and this relationship was serious, he could see it lasting long term, if not forever. Lottie deserves the truth. You can't have a real relationship based on a lie.
Sure, he was scared. He knows how important honesty is to Lottie. And Lottie is so close to Nadya. He trusts Lottie but he couldn't deny he was a little worried this would change the way she sees him.
It took him too long to work up the courage to say something. But he finally did.
That night, after dinner he asked Lottie to go for a walk with him. They were chatting like always, but he was sure she could sense that he was distracted.