HI,
You can be cool. Nothing to worry.
At this age, this is very common when child is expose to super hero concept. ( schema)
It's not a harm, he may get bored in few days.
If you suddenly join in karata or others , he will not join or show interest.
Next Steps
1 Try taking to outdoor, first with children park or animals park or even mall where all children plays.
2.You can also play with him outdoor like shuttle or small foot ball on road or ground.
3. Convey that super hero wants to play outdoor.
Health Tips
1. Don't expect immediate change, it's a process.
2. You and wife be a role model for playing outdoor.
3. don't expect instructions based behavior changes for your child. You also need to on board.
All the best sir....