Very loose,stinky stools

I know is all depends of our puppy,but unfortunately my puppy doesn't get on with Hills at all.
I have 6 months old Old English Mastiff puppy.He is on Hills since I got him from the breeder and as far as I remember his stools always made me think if everything is ok but I did not wanted to change anything to not make his tummy upset even more.
After my observations : his stools are soft or very soft,not formed enough to even clean it properly.The smell of his stools is simply disgusting!He ended up with diarrhoea recently and it took him 3 days to settle down.After feeding he seems to be a bit bloated.And they are days when he goes to toilet very often.

I decided to mix his food with rice or add a bit of different food and he was fine straight away.

I've heard loads of good opinions about Hills but for last few months seems that Hills run down a lot with quality but price is still the same.And if I could add-loads of my friends had same problem with theirs dogs.