It is very difficult to hack a phone. Usually the software needed has to be authorised by the government and is only used against spies and people of note, Like Jeff Bezos. But again it is something that has to be carried out by a Government sanctioned agency.
Here is a very interesting video I watched yesterday about it. Note he is very detailed and technical but he makes it clear how phones can be hacked and it is very specific. And can only be carried out using software that is only available to governments.
I agree that if he is seeing your messages it could be via messages for web if you have that service set up on your PC or his pc. But you would physically have to set it up and it requires your phone to be connected to and in range of the pc to work.
I think you need to talk to your boyfriend if you have such trust issues. Maybe he isn't the right person for you, if you suspect him of committing these kinds of crimes against you.