[Abi]

I have a 10 month old very high energy pup. He is currently in intermediate agility, search and rescue training, and seems to be the equivalent to the energizer bunny. That being said...he isn't eating as much as he should be and is starting to get skinny on me. I saw a GNC calorie supplement in petsmart the other day and was wondering if anyone here had any experience with it or with products similar?
I am running out of ideas...we have switched his food, tried wet food, tried adding treats...it seems like he would much rather be: running, swimming, fetching, jumping, barking, ect than eating...