Yes, it does seem to be normal for cf babies, and I am sure babies without cf as well. But for those of us who take care of cf babies it can really stress us out. The best advice I have for you is to try and not get stressed out about him not eating as much if you can. There is not much you can do to get him to eat "what he should" if he doesn't want it. The more you pressure him to eat, the more he might not want to eat. It is a good time to introduce some foods and let him decide how much he wants to eat.
I really stressed over getting my daughter to eat "enough" when she was a baby and it backfired on me. We still struggle today and she is 7. I have to pick up her fork to get her to eat. For my 2nd baby also with cf, I just let him decide how much he wanted to eat and tried not get upset if he didn't want to eat. I figured that is what most moms do and there kids eat well. He has always been a good eater. Also, the book "Child of Mine" by Ellyn Satter really helped me in getting my son to eat well and me not to be anxious about it all. I highly recommend it.
Sharon, mom of Sophia, 7 and Jack, 5 both with cf, Grant, one month carrier