How to keep in touch with old teacher?

That's not weird at all. I think it would be a great idea for you to stay in touch with him. I would go with the Facebook option first though. Add him on Facebook, you can write him a message instead of a letter. It is more acceptable nowadays. =] But I definitely think that you should add him. I have added my teachers on Facebook. It's normal. They are people, too. Haha.