Hello and welcome to the forum Shearer. I have never had a diabetic dog and as you say, it is unusual in a puppy. I would be inclined to look for a good quality food just the same as for any pup. This means that it needs to have crystal clear labelling and a good quality digestible protein source which should appear at the top of the list of ingredients.
When I use the Dog Food Directory to source foods I always use the clear labelling and no red ingredients tick boxes. This usually brings up the higher rated foods. I agree with spanielowner in that complex carbohydrates might be better as they release sugars more slowly. Brown rice fits this criteria so might be worth looking for products containing this. Some of the grain free products contain white potato and derivates so it might be best to avoid those.
I am wondering if the cold pressed foods which contain brown rice might be helpful. These products are highly digestible and tend to attract a good nutritional score on the Dog Food Directory. Some of them are grain free and if you prefer these then you need to look carefully at the carbohydrate source. Sweet potato would perhaps be better. We have a thread on cold pressed foods here.
There are more cold pressed foods available these days and some are made in the UK. The ones I am familiar with are the Markus Muhle varieties and the UK companies that sell this are Gentle and Guru. Both have versions containing brown rice and have staff who may be able to help you. The big advantage of cold pressed food is that you will not have to change it as the puppy grows up because it is an all age product - you just give higher amounts to puppies and reduce it as they get older. As mentioned, their staff will hopefully be able to guide you on this matter.
Another option (but more expensive) is fresh cooked food. Butternut Box has lentils in the recipe and these are complex carbs.
If you prefer traditional kibble or wet food then you can search for that on the Directory using the filters but please ask if you need our help with this.
I have to say that whatever you decide upon you need to take the ingredient list to the vet to check.