I'm afraid it's impossible to say, because there are many different apps of different types that use heart icons for notifications. Yes, it could be a dating app, but it could also be a fitness app, some social media app, and I've also seen ordinary SMS apps, various TV show-related apps, and other more obscure things use heart icons. And in this case we don't even have a picture of the icon to go on (occasionally there is something distinctive enough about it that it can be identified, though most of the time it cannot).
So I'm afraid that without more information all I can say is that you have to use your judgement of him. Is it plausible that he might not know what a notification on his phone is? My wife's not interested in phones, so if she told me she didn't know what a notification was I'd believe her, but if I told her I didn't know what a notification on my phone was she'd rightly find that completely implausible. How likely do you find his story that he doesn't know?
