Most Helpful Guy
If you two are in an exclusive relationship then I get your concern. This is one of the difficulties of a ldr especially when the couple is split on opposite sides of the globe and you are so young and you only see each other a few times a year. I just don't think its a reasonable expectation to set for a relationship.
Most Helpful Girl
You're not overreacting, I would be seriously concerned too, especially since Tinder is a dating/hookup app. I would just be honest but gentle about it. Let him know that he's crossing a boundary with you and it makes you uncomfortable that he has a dating app on his phone and it is causing you to doubt the relationship. If he can't understand that then he isn't mature enough to be in a committed relationship.