Hi there I am Dr. Jody Navitsky a general pediatrician and I can talk with you about your grandson today.
Is this the only gesture he does repeatedly? Otherwise is he pretty healthy without medical concerns and developmentally on target for his age?
Kids his age can sometimes experiment with tics as a kind of pressure relieving thing, or even just if they are bored. For the most part they are not consciously aware of them and may not be able to express why they do it, or even notice that they are doing it.
At this point if he is otherwise fine and has no health concerns, is doing well in school, and doesn't have a lot of stress for any reason (like divorce, or new baby, or other things like this), AND this is the only repeated gesture that he does you should be safe to watch it for a while. If you want to draw his attention to it, with a gentle comment like "your shoulder's watch your shoulders" when he does it.
This may help him to try to control it.
However there are some illness that have tics associated with them, such as tourettes which may need a specialist becoming involved in order for them not to disrupt his lifestyle.
If you feel that multiple tics are developing (eye blinking, throat clearing, etc...) and they are becoming very intrusive for him you can bring up the issue at his next doctor's visit or consider consulting a neurologist.
There are some occasions where medication is used to help a child control their tics.
Otherwise I would encourage you to either ignore it as long as new things aren't developing, or try some gently reminders. I hope this information is helpful to you. Please respond if you have any thoughts or further concerns that I didn't address. Thanks!