Help!! All the food I give my dog makes him sick

Hello and welcome to the forum.  I am sure you will find alot of good information here and there is already a ton of information on food.  The thing is that here like everywhere else there is alot of conflicting information.  The reason is that you have to find the food that works best for YOUR DOG, and sometimes that is hard, it seems you are already seeing that.

I also went through what you are going through.  I had Fiona on a food that she was doing well on and then I decided to change to other brands because of something I heard, read or someone told me they feed it and it is great.  I went with Blue Buffalo, Canidea and Timberwolf and all of them gave Fiona the runs.  I went back to the original food and have stayed there for 6 months.  I dont add anything to it....just dry kibble and she has done great.  We have solid poop's that dont happen a hundred times a day, a great hair coat and her weight is great.  She eats Purina One Salmon and Rice and Sensitive Stomach and Skin.....would it work for your boy?  I dont know because like before what works for one is not really that great for another.  The bottom line to the dog food is that you buy the best food that you can afford to buy and maintain your dog on, from there a food is only as good as how well your dog responds to it regardless of the scores, surveys or how it works for some one else's.....

My suggestion on the Science Diet is that it has to many things that they dont need and there is probably a better food that your guy will do well on.  There are plenty of foods out there that are "tested" and generally that is not why vets sell or recommend it.  (Science Diet) it is because that is the one food that puts a lot of money into Vet Schools and just about the only brand you learn anything about in vet school.  They generally recommend it because they dont know any better.

If there are no other health issues causing your guy to have a upset stomach (virus, worms) then my suggestion would be to try and visit some sites online and request sample bags and try them or go to a food that is shown to do well with sensitive stomach dogs....

Have you used a probiotic?   Boxers are known to have very sensititive stomachs.   Also as a pain in the ass as it seems, write down the top 5 ingredients in all the foods you tried.  I would make a list of all the ingredients they all have in common.   I would then select a kibble that does not have them or at least most of them.   This was you can switch around by ingredient, rather than by brand.   I would try the purina sensititive stomach.  Its salmon based and potato.  I would also try using a probiotic in the meal to aid in the digestion.   Have your vets done a stool sample to rule out parasite?

All dog food works different on all dogs.  It can take up to two weeks to see a change in the dog as a result of the food.  Also look at your treats as they could be the culprit too.   My Kash has a VERY sensititive stomach.    I have him Pineapple one time, and wow, nightmare for two days.   So I would look at everything you are feeding, sometimes it is not just the kibble.