Geez! If you can manage to go to bed together at the same time, especially during the weekend, can you take turns reading a few pages? Just start reading the book again from scratch, together.
Let him know that you're wanting to work on things and his lack of interest is giving you mixed signals. Remember to start off the conversation with positive comments (like ever since he brought it to your attention you've realized how much he must have felt neglected) and work your way up to how you're feeling... but refrain from suggesting "he's less motivated". Good luck!
