For now without paying for such work the easy way is to factory reset the phone, set it up again and then never allow it out of your sight/control.
-> Current Androids do not have a way to install apps if I know your IP. There are folks that can not be convinced or helped over this and what can you do for them? Not much.
All the exploited phones I've run into were done by the owner or another person holding the phone and installing the trojan.
What are the ways to identify a hacker remotely accessing my phone and files by knowing my ip address. If at all he has remotely connected to my phone and installed a malware like Trojan will he be using the same ip address as mine or a different one whenever he connects to my phone.