9 Week old GSD. Food?

I have been giving him Iams puppy food for large breed dogs 3 times a day. In the morning I mix in a little canned Iams. He randomly throws up. Last week he threw up a lot on one day. Then a couple days ago he did it again. Today same story. First consistency is just puffed up wet food. Then it got a little darker then it was just fluid. In one hours time he threw up four times. He doesn't act sick though. It's weird! Called the Vet and she suggested mixing some cooked rice and cooked chicken or turkey and feed him that - which I have prepared and will do. I'm not sure of the ratio - how much rice and how much chicken? Any suggestions? Anyone experienced this with a puppy? We got him at six weeks - he came from a horrible puppy mill - he's been to the Vet once - no shots but did get dewormed for hook worms. She also gave me probiotics to sprinkle on his food for ten days...I did. He handled that well....he goes back to the Vet tomorrow for a check up and whatever shots/deworming needs to be done.