Hello, you do seem to have a bundle of 'fun' on your hands.! I have a minature schnauzer who had behaviour problems and he too was fed Pedigree -tinned wet food, not dry kibble. As well as intensive training I too looked at his diet and this began my journey into researching quality dog food. I currently feed raw, but he tested many other foods along the way. What I am sure about is that certain foods can effect his behaviour. In his case it effects his ability to control his emotions and focus. I also found that grain free diets are better for him.
I would encourage you to really study this website to inform your choice, I found the information in all areas invaluable.in the meantime a few pointers would be to look for ingredients that start with a meat source, avoid the terms meat and animal derivatives, look in the ingredients part of this site for more detail. Look for food as natural as possible. Beware of supermarket foods although one or two better quality foods are creeping onto their shelves. If you have a local pet shop see what they stock or Pets at Home have some good ones, their Wainwrights grain free has good reports. I personally don't like feeding dry kibble and they have some good wet varieties, Natures Diet, Natures Menu, Forthglade to name a few.
If you find the a Harringtons helps I'd try this for a while and then try something else. I found there is no quick fix and
research can be confusing and daunting- but its worth it. The change in my dogs behaviour and appearance has been wonderful. So do keep trying with your boy. The reward will be worth it.