Puppy keeps throwing up?

Ok, I have a 3 1/2 month old Australian shepherd puppy that my older brother adopted for me from his friend who breed her female aussie and gave the only male in the litter to me. When we received him he had no shots period. 2 weeks later we took him to pet-smart to get his shots and we were told to bring him back in three weeks, and to not let him play in the grass because of something in the grass that is bad. The first 2 weeks were fine then he started throwing up, its light brown watery vomit. I have to admit during the three weeks we did let him sometimes in the grass. I think that's why he is vomiting so much, but I'm not sure. I KNOW FOR A FACT ITS NOT PARVO, he eats 1 cup in the morning and 1 cup at night of pedigree adult complete (my brothers friends told him to feed him adult complete dog food,plus he never vomited eating it so it not the adult food) and drinks his water regularly, and he is NEVER ill looking or laying down and not moving, he wags his little tail a lot, always happy,playful,and he looks really healthy, it's just he is always vomiting. I don't know what to do...please help :(