I do think it might be difficult to get a restraining order but not a lawyer so who knows. But definitely document all the items you receive, document your request that he stop sending them (w/ images, maybe certified letter?). This behavior can escalate, so it's good to have evidence of how long it's been happening. If you move again, get a PO Box. Follow this advice on shutting stalkers out of your tech (some of this may not be relevant if LW hasn't been living with their father but the advice on limiting how much personal info you share in conversations & altering geotagging settings is still applicable).

Also, just want to add that taking these steps is a really awful & painful thing to do. LW needs to do a threat assessment and decide how important being unfindable is vs how distressing or difficult these steps are.