Aussie not eating again

My aussie was born last may. My breeder fed him a purina large breed puppy food. When he was about 3 months old, I switched him to Natural Balance Ultra Premium dog food. He seemed to do well with this food for a while. Last November, when he was 6 months old, we left him at his vet's kennel for a week while on vacation (eating their food). He came home sick with diarrhea that lasted about a month. We ran a bunch of tests on him, but the vet ended up recommending a hypoallergenic food (I think from Purina) for him because she could find nothing else wrong. He did okay on that for about 1.5 months. I decided to buy him Natural Balance Vegetarian food instead of the prescription food after that. He did okay on that for about a month but for a few days he wasn't that interested in eating. I started mixing the vegetarian and the ultra premium together about a week ago and he was very excited to eat. But yesterday and this morning, he turned up his nose at food.

How many meals can my dog skip before I should really start to worry about him. Am I doing okay with my decisions about his food. He's 10 months old now, is this an adolescence thing? Advice would be very much appreciated.

Alan