Eating of grass could be the cause of many things, which are not even proven facts. It may or may not be a myth that they eat grass when their tummies are upset. It may be just because he likes the taste of it. No one really knows for sure.

Your best bet would be to get him to the vet for an exam.

Is he still having an upset tummy? Maybe switch him to a bland diet or a special diet?
At 6 months of age you can probably change him from puppy food to adult food. Nutro Natural is a great food, so is Canidea.

I would also enroll him into a reputable obedience classes ASAP. And also speak with a behavorist about his aggressive behavior.

Good luck with the cutie, don't give up just yet. Be patient, learn & teach, it'll all work out in the end.