That is a good way to actually tell if someone is interested in you or not. When I mean interested, I don't necessarily mean romantically so don't get your hopes up just yet. But when someone likes somebody, whether it's platonic or whatever, they tend to imitate them. It's natural and we do it all the time, usually without even noticing it ourselves. So I guess you can it's good news that he's copying you. It means that he does like you in some way and you can develop your relationship through that basis.
Hope this helps! Good luck!