Certainly ultimatums are not the way to start the conversation, but sometimes they become inevitable at the end of many painstaking coversations over a long period of time.
Sometimes two people are good for each other, sometimes they are not good for each other. In the latter case, it isn't an act of kindness to keep trying to stay together indefinitely. Better is to shoot for an amicable separation, and remain friends.
Only you can decide if/when you've really done all you could within reason to explain your position to your husband, to whatever extent he's willing to listen (and work with you). I don't have any magic formulas for getting a monogamous spouse to tolerate polyamory. You talk, you plead, you explain, you ask them to read books and articles, and ultimately you ask them to set ground rules as necessary and give you the go-ahead to give polyamory a try. If they can't do that, if it's just not within their range of ability, you don't try to force them.
I suppose you could ask your husband to join this forum, read some threads, and post some of his concerns. I don't know whether it would help. There's a Golden Nuggets board
he could look at. It explains a lot of the basics.