looking for help here. My male 18 month old male is really skinny but in extremely good shape. I can not get him to put weight on. I am feeding proplan sport 30-20 and he doesn't seem to overly like it but does eat minimum of 3-5 cups food a day. I just got a new pup yesterday and if I put a little proplan puppy food in with his he eats it right up. Is it the competition for food now or do I continue to put a little puppy food in with his food in hopes he puts a little weight on. Any problems with this idea? He does not have worms and is vet checked regularly.