Honestly, some people just never know if they want kids or not. He may not be ready at all to have them, and just chose to say "i don't know" to the subject of having children. Some never have children, and then some know for sure if they want children or not...like you.
One thing, is never pressure him into it, or the conversation....there is no "time limit" to let him decide, that would be pressuring. Just leave it be for now, and when he is ready, he will let you know.
Sounds like you really want children right now...unfortunately, you and him are not on the same page.
You should never give an ultimatum like "tell me if you want kids right now, and if you don't i am leaving"...just to get him to have children...because in all honesty, you will probably not like his response.
I wish you luck, and believe me when i tell you that you will just "know" if you are needing to take a "different road"....perhaps away from this guy...even though you love him....again...good luck!