I wouldn't worry about it..Bo is 1 year old and only 48.4 lbs...when we got him at 9 months he was 46 lbs and a bit skinny...took him nearly 3 months to put on any weight..he's just soo active...peanut butter in kongs are a good, high protein snack...and like lisa said...eggs added to the food...yogurt, etc. are good. All boxers are different and all grow differently. Jetta is almost 7 months old and is only 38 lbs...so both of mine are very "small" for boxers...but both look and act healthy. Bo has all of his ribs showing slightly (vet said this is fine due to his activity level and how much muscle tone the rest of his body has) and Jetta you can see about 3 or 4 of her ribs, which again the vet said is normal due to growing and activity. So I really wouldn't worry about it too much, give him some supplements and I would highly recommend a new food, Purina isn't a very good brand and has lots of fillers (which is why you have to feed him so much). I had mine on Canidae and was very happy with it...they only needed a total of 2.5 cups of food each on that- no corn, wheat or soy and has only "good" usable ingredients, so the dog uses it all and doesn't have a bunch of crap that just makes poop. I switched to Diamond Naturals *a little cheaper, but still no fillers/corn/wheat/soy in order to up the daily amount of food given to fatten Bo up* and now we are switching to another food to try to find something that's a little easier on the gas and Bo's coat. They say it's a good thing to rotate foods to help prevent food allergies.