Should have caught him red handed by grabbing his arm that time.

We can talk about all possibilities about intentional or unintentional bla bla bla.... let's skip that, the guy definitely know that his elbow touching on something and it's very abnormal to not wondering & do something when one of your body parts is touching someone or something.

Report to police is an option but it's easy for molester to deny. Unless you confront that person with your friends around and make him admit that

You may post the shop name to alert people about it