I feed Royal Canin; in fact it was one of the only foods my finicky eater Joey would eat. He's not as finicky as he's gotten older, but he's done really well on the food and I believe they have a great quality control. I do agree you should keep some food out all the time at this age. They can easily run out of their energy reserves and food should be available 24/7 until at least 7 months. I think you are making a mistake by hand feeding, you are teaching him to become too dependent on you, and this isn't good for your dog's self-confidence. If you are going to hand feed, it should only be done while training. Go easy on adding the chicken and carrots, if you are adding too much your dog will reject his kibble and chicken and carrots aren't a balanced meal, and don't contain enough calcium for a growing pup. If over 1/3 of his diet is coming from other forms, his diet is no longer balanced.
Do a google on Comfortis and you will find many dogs get lethargic on it. I don't use flea medication, so I can't advise you, but your dog might be sensitive to the product and/or too small to use it. I'd ask my vet about this. They go through growth spurts, so it's okay if your dog isn't gaining every week, but I would be worried that the flea medication is too strong for him. Are fleas really bad in your area? Are there other things you could use? You might want to check out this article:
Comfortis side effects? - Dog Health Forum