Sorry you're dealing with this. The little ones go down hill so fast. Many times they're too far gone by the time you even realize there's an issue. Hope you're able to help him pull through.
So sorry you lost your boy.
I would try injectable Ivermectin given orally, 1cc / 40 lbs. and get a fecal done before and after. Also give them Red Cell. Safeguard has to be used at higher doses than what is on the label and I don't recall the dose. 1cc/10 lbs??? @OneFineAcre, is that right? Pelleted wormers are hard as they might not eat as much as they need. Valbazen I have no real experience with, I just started using it for Tape worm last week.
I don't think that there is any way to get rid of worms in the pastures unless you can leave it empty, rotational grazing.
Once you get the worms under control you might try adding an herbal wormer to their diet. While I am convinced that it won't get rid of them, it may help with them coming back. Just a thought and it can't hurt.