I think you might want to do some research. When babies become more active they often don't gain as well. My oldest and Jace each had a period they didn't gain at all. With Jace they wanted us to supplement with formula and I did a few times but it was not what I wanted for him and I kept at the nursing and he is still little (3rd percentile) but fine.
Doctors are not educated enough about breastfeeding and they look to a quick fix to answer for kids that don't follow the norm. Well not every baby grows the same way. I would say as long as he is growing and gaining (at his own rate) and meeting milestones he is FINE!
I did a lot of reading and talking to other moms with these problems when jace was first born because I was so devastated. I basically got from the doc that breastfeeding was hurting my baby and we had to stop. Of course I decided that wasn't the way things were going to go. We went for weekly, then monthly weigh ins to make sure he was still growing. (I actually skipped the last monthly weigh in because I'm truly over it!) I even rented a scale to weigh Jace before and after a feeding and mark down how much milk he was taking.... which was plenty ~ go figure.
Like I said if he is healthy, growing and meeting milestones I wouldn't stress too much. Feed him what he likes (baby food, puffs, breastmilk) and let his body do its thing.
Now for the vitamin I would totally give that if he is deficient.

LOL
Did I rant enough?