I don't think you get it. If you are going to lead, lead by example. Your intern is coming in 5x a week, you go ahead and come in 5x a week too. Fair is fair, he will be just as accessible as you during the WFH. In regards to your bullet points, lets go through them:

1. The future of flutter: it kinda sucks. You can make apps that look cool, but don't tell me is a great cross platform solution. If you have dug down in flutter, using channels and using native features fully, you will realize it falls short. Their bluetooth is the worst, I built a cross platform mobile app in it for a full MVP prototype... and I realized why? I liked the Skia engine, and ended up using Delphi with FMX + Skia to build a much better application that is cross platform with better support. Sure you need to pay for the IDE, but I do so too when using Unity and VS.

2. The purpose to learn a new language/framework: there are plenty of garbage languages to learn. Sadly you never used C/C++/Java on real software, I have. Personally Javascript/Typescript is something you just pick up as it is a scripting language.

3. Apart from the technical stack: some software has gone through so many hands and not consistently that it barely works or compiles. Your intern can get hands on experience on his own projects, taking side projects, or contributing to opensource projects.

4. The purpose of the internship programme: he is interview your company too, dont take it personal.

5. The "Work from home" policy: be fair, your company seems like it is not. If you are working from home have them do so as well for the days that you are not in the office... make this into a team.

Conclusion: you lost a smart intern that knows his own value. Those are the keepers and hard to come by.