I dunno... I used to feed Gonzo Nutro when he was a puppy and he barely ate it, and once I switched him to a higher quality food (Solid Gold), he cleaned his bowl every time! Iams is not exactly the best quality food (corn, wheat, fillers, by-products, etc)... he might not like the taste or it might upset his tummy, he could be allergic to all of the fillers. He might also just not have a big appetite in general

If you're concerned about his weight, by all MEANS find a really good food and gradually switch him. Chicken Soup For the Soul dog food has said to be loved by dogs, and I know Gonzo LOVED Wellness! Wellness by Old Mother Hubbard is an amazing dog food with human grade ingredients, and it even smells awesome! Any high quality kibble (I suggest Solid Gold, Wellness, Canidae, California Natural, and Wysong) would definitely put some weight on him and give him even more energy and health!! You will also see improvements in his coat, teeth, and skin on all of these brands. With Canidae, which I know tons of Border Collie people including BCBlondie feed their dogs, it is so high protein and high quality that you only need to feed a 50 lb dog 1-2 cups of kibble per day. That means, you'll be feed around 50% less food.... although Canidae and other good kibble DOES cost more, it will actually SAVE you money because you won't have to feed near as much. You have to feed him so much Iams because it is full of wheat and soy and corn, which do not put weight on a dog or really have any nutritional value.

good luck with Blaze, BC's *should* actually be on the skinny side, as they're working dogs and are bred to be energetic. But if you can feel most of his ribs and/or his hipbones, he's too thin... it isnt a big deal on dogs that are still adolescents, as BC's tend to look like they're starved until they're about 18-24 months old, but Blaze is 3 or 4 now and he should be filled out. It's awesome that he gets plenty of exercise!!