I'm with aly, I would check into the legality of what he is doing. It sounds like he would be changing a lease mid term. Just a suggestion and it sounds like you are on good term with him and making it a legal battle may not be the best idea. He would also have you next time te lease comes up for renewal.

You might want to see if he could do something, like make a grandfather clause and let Moose stay while keeping new pets from comming in. That would be a way to handle it and still be fair to all. I don't know if you would be willing to move, but you can ask about grandfathering moose in and back it up with the a threat to move. However if he says no you would be in a postition of having to movw.

I really wish I had a better suggestions.

Well however it turns out at least Moose has a good place to stay. That is a bright spot.

Let us know what happens.