I am so thrilled! Wow, all he needed was his couch!
Everything you are doing sounds wonderful and yes, I would definitely encourage him to get up at times and participate with you once he is motivated. I want to suggest that instead of requesting him to get up and have a conversation with you when he gives you a green light that you entertain him first by building an activity that doesn't require him to do anything first.
When we get a green light, we either build from what he is doing or initiate something new. It will help him to build more of a rapport with you if you first have fun role modelling a game for him or entertaining him by doing a fun action and then when he is looking, smiling, paying attention, appears really interested in what you are doing (or any of the above), I would then invite him to physically participate.
I would need to know more about his language ability before recommending you start a conversation with him but in my opinion, it helps any child I'm working with to have them get up and moving and encouraging them to have some kind of role in the game. This helps them connect and invest more in the game and then I can add some conversation once I have an interactive attention span going. For example, he lies on the couch and then looks over at you as you join him. You could try jumping up and playing a harmonica for him, then as he continues to pay attention, add to the game by starting to dance as you play, once he is motivated, invite him to bang a drum with you while you play harmonica, or to dance with you.
If he enjoys it and plays the game with you, you can then go to working on language (for example, ask him which instrument he wants to play next, what song to sing, etc).