It's unlikely to be the protein content. High protein content is fine for pups/dogs as long as the protein is coming from a high quality source. Lots of feeding guides recommend 23/4% protein for pups, but they are usually based on feeding lower quality kibble, which won't have such a good quality protein source. Anything that says 'meal' or 'meat/animal derivatives' contains poor quality protein, if it lists a large % meat first in the ingredient list it's much better quality. Which Acana is he on?
You can buy probiotic pase online. Our vet gave us Canikur for Pip and we've sourced it cheapest online here. It's just a 2ml dose twice a day for dogs/pups under 10kg and you just use it for a few days to 1 week in most normal cases of upset tums.
I would say perhaps you need to keep him off complete food for a little longer, perhaps 48 hours, then gradually reintroduce it, rather than stopping and then going straight back onto a whole meal of it. Starving isn't advised these days, as it just gives the bad bacteria chance to get a stronger hold. Small regular meals of easily digestible food (so boiled or oven cooked chicken without the rice, he doesn't need the rice) will probably sort him out in no time and probiotic paste will just help him rebalance his gut flora.
Pip is still on 7/8 feeds a day and was on purely cooked chicken for the first three weeks, but is now being transitioned onto Canagan and tolerating it well. He hasn't had a full meal of it yet. I started off just using it for training treats here and there, then gradually increased the amount he got. Today he's up to having about 10 kibble biscuits added to each of his feeds and we'll gradually build that up this week.
Just spoke to the vet. Pip has to restart his vaccinations, so that's completely kyboshed his whole puppy class course. 
Going to have to get in touch and see what we can do about it, as attending a puppy class course is a condition of his adoption and we have to provide proof he's attended.