I am an undergraduate. In my internship last summer, I did my job well and I could feel that he genuinely liked and recognized me. He was great and I was in a great team. However, I always want to try different things so I accepted an internship from a different company for the next summer. I have told the recruiter from the last company and my mentor should have already known that I would not return. However, should I still send an email directly to my mentor asking about his recent days, telling my plan, and etc.? I don't want to just lose connection with him, but it's obviously a little bit awkward as I will go to a different company. So could I know what is the proper things to do now? For example, should I just send him an email and be grateful? Or is being slient the most appropriate action? Thank you very much.
Usually, not much can go wrong with a simple and polite 'Thank you' - Email. You'll usually not burn any bridges, unless your mentor is some kind of jerk, and most people will see it as the polite way to handle things.
If you want to stay in contact, mention it. Just accept it if your mentor decides otherwise.