M., he has found your button and he's not only pushing it, he's punching it. As long as you bring him in your bed, he will do this. As long as you nurse him in the middle of the night, he will do this. You were able to successfully sleep train your daughter partly due to her disposition and partly due to the fact that there wasn't another child to wake up. Your second child is harder on both counts.
Here's my suggestion. It's going to be hard, but you and your husband need to look at the greater good here.
If there is a place you can put your daughter in the house where she cannot hear him cry, move her bed. If there's not, get her some little ear plugs and put them in her ears while she is asleep. Then, no more going in period. Cold turkey. It will be hard. It will be brutal. He'll scream as if there is no tomorrow. But don't go in. Once he realizes that you aren't coming in, he will gradually lie down in the crib and fall asleep. The next night he'll cry a little shorter time, and in successive nights, he will finally stop waking. It may take up to two weeks.
It will ONLY work if you are 100%, and I really mean 100% consistent. If you cave in and bail him out, going in his room, he will know that the gig is up and he has won. Then he will scream to high heaven.
Don't start if you can't finish. If you give in to his crying, you might as well have not done it.
He does not know how to self soothe. He uses YOU to put himself to sleep. He doesn't need to nurse during the night. It is a habit, not a need.
Do him the favor of allowing him to learn to put himself to sleep. It will help your entire family.
Good luck, and sending you strength.
Dawn