Sorry to read this, it is an awful illness. I will tell you what I did with Brodie, when she had it over a year ago. It can take some time for the enzyme levels to come back to normal, so in that time she was on Royal Canin's gastro-intestinal wet food, which is just 1.7% fat. She was allowed no treats or chews during that time. I believe NatureDiet (a wet food) have a fish version which is 2% fat.
Once her levels were back to normal I moved her onto NatureDiet's senior/lite, which is 5% fat. My usual guidelines is 5% fat or less for food, and around 1% fat for any treats or chews. She's been left with a very sensitive stomach which limits what I can give her, but I know of other dogs who have went back to having normal diets with no further problems. I will be keeping Brodie on low fat food for life though, to prevent a reoccurrence. I also have to watch her when my niece and nephew are here and there's food around she might steal
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Really hope your boy has a quick recovery, it really throws them and is such a worry