This is a great question and one we geat quite a bit!! It sounds like you are already doing a really excellent job of picking up on his cues and realizing that it may just be more about the answers at this point, than actually building a game. My suggestion is really dive into his topic at hand and answer answer answer... keep going until you find that he gives you a
stronger green light. This may be a pause in the interaction, or a longer look, or a change in his affect, but know that above all, you have a big bag of building goodies to use when he is available... now its just about finding the right time to use them
Have fun with it, enjoy the things he loves to talk about... he will be completely aware of this and then become that much more likely to want to interact with a deeper sense of connection later.
1. Relax and enjoy the questions
2. Have fun answering... even if its the same answer- know that each time you give that answer you are building a deeper relationship with him
3.Trust that he will show you when he is ready to allow a change or move to another activity (for a lot of our higher functioning children the questions or perseveratory topics are isms just as you are describing ... because really all we have room to do is answer or listen, there is room for nothing else we are predictable in this way and helping them gain a sense of control in this world)
You Rock Son Rise Mama!!
Keep up the great work (play

) and let us know how it goes!
Big smiles,
Kim