Not all treats are fattening - the processed ones tend to be; and they are usually loaded wih additives that can turn cocker puppies into manic little toddlers on E-numbers
As Rachel says, vegetables are good; buffalo meat, cows ears, hooves and pizzles are all low in fat and natural so no nasty additives
I always use "high value" treats for training reward, and then lower value, lower fat treat for everyday reward
So, because we care currently working on recall, I use tiny slices of hotdog sausage as recall rewards; with veg, gravy bones and pizzles as rewards and treats the rest of the time
Remeber you can always reduce his daily food allowance to compensate for treats - I always did this if he had a pigs ears to eat as they are really fatty