There are several possibilities, depending on just how careless one gets. If he links his appleid with the new phone, it could show up there. Check FaceTime to see if there is suspect history there. Most likely, the new phone would be a burner with pre-purchased service.

I would get ahold of the iPhone you know about, go to Settings:Privacy:Location Services:System Services:Significant Locations. Look for something in there that looks suspicious.

married for 40 years
DS - 38, autistic, lives at home
DD - 37, married and on her own
DS - 32, still living with us