Would this be repetitive behavior?
DS has recently started looking up and even upside down, bending backwards to look at things. It's similar to the way he'll lean backwards when in our arms so he can see things upside-down. All of my NT nephews did that as toddlers, but I don't remember them doing it while standing on their own, or with such frequency and unwillingness to stop.
Also, DS has started to "ask" people to sing by tapping our mouths when he can reach. He waits for the end of the song, then does it again. A LOT like he does with his glow worm. 
Should we be discouraging this? He seems to be treating people as objects instead of interacting or communicating, but we love singing to him, and he really only makes eye contact when we sing. His developmentalidt has encouraged us to do lots of song based games, if that helps. She says it's the key to his ability to interact.
Thoughts?
Re: Would this be repetitive behavior?
It could be, especially if its obsessive or difficult to intervene.
As for the mouth tapping, I personally would not allow that - I'd teach him a different sign for him to ask for songs.
Sorry, I didn't give you any background info, did I? DS is two and has been evaluated by school psychologist who says she'd feel comfortable with a PDD-NOS diagnosis, but is hesitant to diagnose this young unless it will help him get services he couldn't get otherwise. We're on two different waiting lists to see development pedis. He's always pretty happy when he's doing the leaning back thing, which, for him, equals easily diverted to something else. Left to his own devices, though, he'll go back to it. The singing thing, I agree is a problem. He needs to not treat people as his own personal objects/playthings, but ask for what he wants appropriately through language of some kind.
Actually I have been trying to get my son to tap my mouth for me to sing more. According to our amazing speech therapist any type of communication should be encouraged. Sounded counter intuitive to me at first, but it's crazy how well it has worked. His language has literally doubled since we started seeing her. She really encourages non verbal communication to get back and forth interaction.
I am not sure the repetitive behavior thing. To me I feel like my son is developmentally delayed so when he does typical kid stuff just a little later than I would expect it I try not to worry.
Yeah, he's 2, and given how easily redirected he is from the leaning back thing, I think we'll keep an eye on it for now and not stress unless it continues for more than a few more months. I honestly think it might just be him testing the limits of gravity.
Thank you, Auntie for the info on ToM. As scary as all of this is for me, I am finding the research I'm doing and education I'm receiving to be absolutely fascinating. At what age do/should PECS come into play? He signs a leeetle, mainly for breastfeeding, up, and sometimes more and all done, but really, he always has to be prompted to do that instead of whine and manipulate us physically and his sensory issues mean that he hugely resists hand over hand work.