I want him to stand on his own two feet and be able to budget properly on his own
That's never going to happen. He couldn't do it when you two were together, nothing will change. Accept that. Clean cut and move on. It's about damage limitation now. Believe me cutting off a parasite can be tricky, but it must be done or he will continue to bleed you dry.
ok, so I am going to tell him that if he wants to keep the phone then he pays the bill from now on and he needs to sort out his own money as i can't support him anymore
Nope. Wrong tactic. Take your property from him. The phone is yours, not his. Don't discuss, his word is not worth a piss. I did stupid things in the past with a similar man. Bought a car in my name (due to his piss poor credit rating) which he drove, then left open and let get robbed blah blah.....Look to cut a long story short. He is not reasonable so don't focus on what he says, what you say etc. It's about action now, not words. His words are meaningless anyway.
It is just very hard as makes me feel like I am being unreasonable or that I am in the wrong.
So what. You've got to stop caring what he makes you feel like. Believe me he is laughing up his sleeve at you. He knows well he can manipulate you by playing the poor hard done by one. Stop falling for his little game. You need to learn to accept feeling a little bit uncomfortable for a short while. The fear of feeling 'in the wrong' or 'unreasonable' are the chains that are binding you in this ridiculous situation.
Get rid of him. Let him be unpleasant/play his violin all he likes. He'll say a load of shite, that's predictable. But stop caring. Take control of your life and scrape this cocklodger off. Seriously, you will look back and kick yourself for being such a walkover one day!