Did your husband say you can't have a boyfriend after he found out his ex was only interested in being friends?
I agree with Quath to a point. But it sounds like your husband fell into poly because he still loved his ex which he may consider fair game because in the end we all love our exes to a point, and knowing he had that, maybe felt guilt and let you have some cake to.
He may not be poly, and you may have to decide if you are or want to be. You may have to give some room to let your husband heal from his breakup. With everything being so fresh he may need some time to process his own hurt before he can process the hurt of you having a bf. I would definitely work towards a compromise in regards to your bf, at least staying a friend. I, personally, think its unfair to give room to move and then close back in again. But you need to know your own husband and know where you can make those compromises.
Also consider this, about his depression. He may have seen something in the setup that looked like it could be the perfect setup. By his ex breaking it off his expectation of perfection could be broken. This can be quite devastating. He may need time to heal before he is chipper and happy. And you can probably expect waves of emotions too.