Do not open source this... You've created a business not a library or a framework.
I have no idea what the intentions of the GP were but you should definitely retain ownership of your code. Try to sell it. Try to get ad money. If nothing else just have fun with it.
It's also crowded market with lots of free alternatives out there already. Not everything posted as a Show HN is a business opportunity waiting to be exploited.
I'm not saying this should be open sourced either though. In fact it "shouldn't" be anything. It's something created for fun. All this pressure placed on a 15 year old to either open source or create a business out of it is insane. Just let the kid have fun like we did when we were hacking stuff together at 15.
No, they most likely have an open core model where advanced and business oriented features are typically kept as a separate code base with distinct licensing.
GitLab as an example keeps them in the same repo but separates the structure so business features are clearly and distinctly licensed under a business license.
I don't know of a serious business that has a completely open source code base.
I have no idea what the intentions of the GP were but you should definitely retain ownership of your code. Try to sell it. Try to get ad money. If nothing else just have fun with it.