I am poly, switch, slave sooooooooooooooooo here goes.
As others have pointed out it DOES depend on your dynamic as to just how to proceed.
That said. He promised you he wouldn't bring anyone else into the relationship until at least a year had passed, correct? Respectfully call him on this. Let him know just how upset you are that he didn't follow through with a promise, how it has hurt your trust and how it is making it that much more difficult for you TO trust him.
Let him know you did NOT appreciate the goings on on your birthday as well. To my mind that is a time JUST for you and select others. NOT a time for him to collar someone else, have sex with someone else & pay no attention to the birthday girl. Explain to him why you DID leave, expecting to be able to come home & have the evening continue as planned. Explain that this, too, has damaged the trust.
Ultimately the choice is yours. Collar or no, contract or no, the choice IS yours as to whether to continue this relationship or not. In the end you have to be true to YOURSELF. It is your happiness that you must look after (if you're not happy there's no way you'll be able to make him happy line of thought).
There are as many ways to do polyamory as there are people practicing it!