I am sorry to hear you are worried about your dog. My name is Rebecca. I am a veterinarian and will do my best to help.
Do you know what weed killer was sprayed? Most weedkillers are not toxic to dogs, any toxicity is gone as soon as they dry on the plants. If he is vomiting, it may not be the weed killer.
Dogs often eat grass when they feel sick although we are not sure why they do. It is more likely he was feeling sick already before he ate the grass. Vomiting and mucous diarrhea can be from many things: eating something that is indigestible or spoiled, parasites, infections. Many of these episodes pass on their own in a day or two but some can be more serious.
If he is vomiting a lot, do not try to get him to eat or drink or take medication. If the vomiting slows down, try to get him to drink liquids but in small amounts of water, broth, Pedialyte or Gatorade or ice chips. You can try famotidine, 10 mg twice daily (brand name Pepcid AC) for vomiting, and Pepto Bismal or Imodium at half the adult labeled dose, for the diarrhea.
If he won't eat or drink at all, if the vomiting does not slow down, if his gums appear pale rather than a normal bright pink, he may need medical attention.
Let me know if this helps, and what other of your questions I can answer.