Hopefully, he will be a normal person and react by apologising and say he won't do it again AND actually make the change.
But if he doesn't........Do not see this as a discussion, this is you stating your new reasonable boundaries not a negotiation. Do not enter into a debate. Do not give him a chance to twist it round to his 'insecurities' or his 'anxieties' so he can play victim. Do not allow him to make his insecurities your fault. If he has insecurities they are his issue and his to deal with - with professional help if necessary. Do not start to accept that you can somehow help him by adapting your new boundaries or loving him enough.
Have stock phrases planned and stick to them, on repeat if necessary. Be clear in what you will and will not accept in the future. Be clear what you expect going forward.
Remember that you have been 'brainwashed' into thinking you make him like this. He will do his best to stick to this agenda. Don't accept even the tiniest bit of responsibility because that will open the floodgates and you'll end up feeling it's all you.
Do not apologise, for your feelings or actions. He'll see that as an admission that it's all on you and pounce on it.
Do not allow him to persuade you that if you "really loved" him you won't want to go out alone. He may try to make you feel guilty for wanting what the rest of us regard as normal - a bit of independence. Dont let him persuade you that it's only because he loves you soooo much that makes him want you not to be anywhere without him. That is NOT normal love, that possessiveness.
Let him rant, cry, sulk or storm off. Do not change your expectations or start to negotiate if he rants. Accept he is crying because he's trying to manipulate you, not for whatever bollox reason he gives you. Do not try to get him to stop sulking by trying to get him to talk more, or by being extra nice, by begging him to speak to you or by apologising to him to get things back to 'normal'. Do not chase him if he storms off and do not text or ring begging him to come home.
Do not capitulate to get things back to normal because that normal was fucked and you don't want to go back to it! . Realise this might get worse before it gets better. Short term worse is ok!
Read up on emotional abuse and control. Arm yourself. There's a script, so be prepared to recognise it. He's not special or different. He'll come out with the same shit the rest do.
And remember there are billions of men on the planet who do not act like this. You are NOT stuck with this one. It's not a choice between him and loneliness. It's a choice between being controlled and being happy in the long term.
I really hope I'm being overly pessimistic here and it's all fine. Fingers crossed I'm an old cynic! 