What did his latest glucose curve look like?
How high and low are his numbers going?
Has the increase in thirst decreased?
Does he have diarrhea?
Has the vet recently tested any other bloodwork other than glucose? Like thyroid? Pancreatic enzymes?
I suspect that his glucose is still not controlled. Most diabetics are checked more often by having glucose curves performed. If his numbers still travel too high then you will not be able to get weight on him not matter how hard you try. I would talk to your vet about getting him set up for either this or a fructosamine as his insulin may need to be increased. Once he reaches a balance then his weight should start coming back on . The goal is to keep his numbers below 200 steadily throughout the day.
Types of diets you will want to feed him are high protein diets. Pedigree is actually pretty low in protein and full of complex carbohydrates which is not good for a diabetic animal. Since he does have issues with diarrhea intermittently you might want to consider picking up a prescription diet for diabetes called Purina DCO.
If prescription isn't the best option for you then I would consider getting him started on a higher protein puppy food canned or dry. The food should best be higher quality like Wellness or Eukanuba as the other cheaper foods are just loaded with fillers and aren't very nutritious.
Definitely have a glucose curve performed, I would also suggest rechecking his bloodwork especially pancreatic levels and thyroid. Once adjustments are made and he is stabilized at more normal numbers his weight should significantly improve.