Is it possible to access another phone from your phone?

My son has been acting secretive with his phone lately and I’m worried about who he’s chatting with so I want to monitor it remotely from mine but don’t know where to start.

No. Legitimate tools for remote monitoring are restricted to parental control apps with explicit consent, and these typically require installation on the target device. Any product or service claiming to offer full, secret access to another phone is likely fraudulent and may be illegal.