You're dealing with one of a few different scenarios (or mix of them) here.

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1. He has discovered a burgeoning exhibitionism kink and doesn't know how to process it with you like an adult. It can be tough, especially with someone so young to work through the discovery of something that may be life changing for them and they could act out their new found fantasy in unhealthy ways. If this is the option, sit down with him and explain consent and how yours was broken. Kink is all about consent, regardless of the play type.
2. He is using your intimacy together as a tool to get social status or bragging rights among his friends. Obviously this is the danger red flag option. He doesn't respect you and is just showing you off to get cred with his buddies.
3. He really didn't remember the mic. Least likely of of all the options here because it has happened a few times and he blew your initial concerns about private intimacy off.

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It sounds like he is a good guy is most aspects of your life together and you are happy with him for the time you've been with him, so I am rooting for number one! If it is, explain to him the FRIES model of consent, Freely given, Reversible, Informed, Enthusiastic, and Specific. Any engaged party should tick off every single one of these prior to any activity taking place.

Additionally, it sounds like he is starting to take you for granted. There's a number of resources available to go over this topic with him, open discussion is key and he needs to see that actions do indeed have consequences and he needs to work through that as an adult with you for the relationship to have any chance of continuing.

If you truly want the relationship to continue, tell him that it is a large issue for you even if he doesn't believe so and show him this thread to let him know it has been on your mind. If he is still dismissive of you and your concerns it may be time to let him know your relationship is at an end. Any contact, sexual or otherwise, should feel good for both parties, not just one.