I like your idea to take him to the cinema. You have no idea how much he is being bullied 'off-line' and what kind of things might be happening to him. Most children who get bullied are very secretive about it. Give him a chance to open up to you in a comfortable an informal setting.
Telling his parents is probably a good idea but let him know if you plan to do that and get his involvement if you can. The more he is in control of how this is handled the better.
I wrote an article a while back. It's aimed at parents but you might find it useful. It's on how to talk to your child about bullying.: http://www.bullyingfullstop.co.uk/html/bullying_advice_for_parents.html