Re: Fatty milk
I would just continue to nurse. Your breastmilk has more nutritive value than any foods you'd be able to give him right now. While feeding him starchy baby cereals might pack on pounds it would not be the healthiest way.
You can't judge your breastmilk's fat content by one bottle, there is a lot of variation in pumped breastmilk and it depends on many different factors that are constantly changing.
If you want to try to make sure he is getting more of the fatty hindmilk you could encourage him to nurse longer on one side, try breast compressions to get the milk flowing again when he seems to lose interest, start each session with the breast you ended on the previous session...
but really it sounds like there is nothing to be concerned about and your doc is needlessly worrying you over something she herself said wasn't a concern. Babies grow at their own rates and they aren't going to grow consistently.
“We are not put on earth for ourselves, but are placed here for each other. If you are there always for others, then in time of need, someone will be there for you.”