He is at the age for being leggy and thin! Teeka is almost 10 months and very thin......I would love for her to fill out, too! It really depends on their activity level as to how much you should feed. If he still acts hungry after he is fed, give him a little more. He might have just gone through a growth spurt in height and he just needs to catch up in the weight department. Your vet can better advise you as to him being underweight for his height, bone structure and age. Let us know what your vet thinks.....