Next time he touches you, you could just say 'humm that's nice' just to let him know that you like it, then he might do it more. I think he probably does like you or he wouldn't seek you out and touch you. Brushing against by 'accident' is one thing but deliberately rubbing your back for 30 seconds is quite another thing.
If you are feeling very couragous one day you could even rub his back and see his reaction to it.
Hope it all goes well for you
