I’m trying to keep an eye on my son’s online activity now that he has his first phone. I’ve looked at a few options but want something that stays hidden and actually syncs in real-time.
Hey Liam, welcome to the forum! For a first phone setup, I’d honestly start with Google Family Link - it’s free, built for exactly this, and gives you app control, screen time limits, and location without any sketchy side-loading.
The “stays hidden” part is a bit of a rabbit hole honestly - most legit parental control apps work better when your kid knows it’s there, saves a lot of trust drama down the road. 
For “hidden + real-time” Android monitoring, mSpy is one of the most commonly recommended options—its dashboard is built around quick visibility (apps used, web activity, messaging depending on availability) and it’s designed to be lightweight on the target device. That said, the downside is it can be legally/ethically sensitive depending on your situation, and “fully real-time + completely invisible” isn’t guaranteed across all Android versions/devices (some manufacturers/OS updates clamp down in the background).
If you tell me your son’s Android version/model (and whether you’re trying to monitor messages specifically), I can suggest what to expect from the setup.