Accountability seems to be a common theme here.
He needs to be accountable for his desires, and in doing them in a respectful manner with the people he is already in a relationship with. ( that being you. )The ultimatum sucks,...as ultimatums usually do.
You need to be accountable for the fact you don`t drive, don`t make friends easy, etc. Those aren`t anyone elses obstacles but yours. You need to decide to get past those things, not expect others to cater to them. (or let it be a excuse to yourself)
If you do these things for yourself, you won`t feel as dependent on him to make friends, and be in this new city. The world will be your oyster, and you will feel more interested in putting yourself out there.
As far as him dating 4 women. Well, it depends if he is being honest. If he is, then this is most likely a good thing, no ?
When we are single and dating,the healthy thing, is to go on a few dates, with a few different people. Experience different personalities,..when 1 person draws you in,..then go from there. When poly, it would seem acceptable to date just as if you were single.
Meet new people and 'date' rather then just attaching to one person right away.
As far as the newfound jealousy,..look at it this way ; If he is actually wanting to seperate from you, and all this is some falsehood,.....Well guess what ? Being jealous is not going to help nor change it. It will only drive you crazy.
Might as well look after you, do things that make YOU feel worthy, and confident, and let the cards fall into place.
You can only fix, or mend, or grow in a relationship, as much as the other people want to equally contribute. Make sure you are both doing your share.
Best of luck. You sound like a smart, attractive girl,....keep your chin up. Remind yourself who you are.