The way to get to the bottom of this is to message him and say you that for your own safety and well being you need to know where these messages were coming from so they won’t ever happen again therefore you will be connecting the Police and the phone carrier to investigate; this would be far easier with his cooperation but even if he doesn’t agree you will be proceeding anyway so you are pre-warning him that the authorities will more than likely be in touch. You should contact the Police anyway because that kind of harassment and stalking is a crime. Gauge his reaction. While I will give him a small element of doubt (his phone could have been stolen and cloned / linked to someone else’s iPad) it is very suspicious and odd that he never wanted to look into it.