Does your dog have a problem that makes you limit fat other than gaining weight easily? As long as the fat % is no more than about 1/2 the protein % I wouldn’t consider the food to be high fat. Whether you need to take the fat level into account depends on how much of it you are feeding and if your dog has any health problems that limit the amount of fat he should get.
You say he does not do well on grain free foods. Some dogs that have been on the same food long term have trouble switching because their body has acclimated to the food that it has always had and they actually have to grow the normal gut bacteria back or in different proportions than what the old food would support. Many dogs need a little extra support during transition in the form of probiotics and digestive enzymes.
