You should demand that he cease his inappropriate behavior outside of work. Go to HR if necessary.

That is just so inappropriate and freaky all at the same time

Sexual harassment might have him quit...but that's probably a bit too far

Creepy boss :/

Ask him how the hell he would know you were on a dating app unless he was on it, too.

I think this kinda goes without saying but take screenshots or print out copies of the message(s), that way you can take those into HR or the appropriate people at work and show them where you stated you were uncomfortable with the situation and how it escalated from there.

If your boss starts berating you for being on a dating app I'd point out to him that he found you while he him self using the app and ask is it the use of the app or the fact that you turned him down which brings up harassment issue and it's his behavior inappropriate.

Save the message, have a meeting with HR

Go to HR immediately. Once you told your boss that you found it inappropriate for him to do something sexual/romantic that is the end of it. If he tries to do anything, including calling you in to his office to discuss the matter further there he is probably creating a hostile work environment.

That sounds like a harassment lawsuit.