I EBF until 9 months (plus solids obviously), and am still breast-feeding now. He might get 2 or 3 formula bottles a week so I guess he's like 90% bf. I do think bf babies gain quicker than formula babies in the first 6 months, but then gain slower in the 2nd 6 months of their first year.
I don't think weight loss is normal, but your dr would know your baby, where he is on the growth chart, etc. If he has been sick or starting to get really active by crawling or walking he might slow down on the gain? Did your dr have an explanation as to why he wasn't concerned?
DS has not gained weight well, but he's kept to his curve (at about 17 percentile). He is crazy active though, he probably burns a lot of calories. I am always worried about it and try to feed him high fat foods like avocado, cheese, etc. I also give him a multivitamin because his iron levels were low (dr.'s recommendation).