I went through a similar experience with my pediatrician over my daughter. She progressively gained weight until between her 12 month appointment and her 15 month appointment. The concern was her fall ont he growth chart by not gaining weight. My pediatrician had me start Pediasure (which my daughter hated) two times per day. I also asked if there were things that I could do in addition to that. The suggestion was to prepare my child's foods with butter, not margarine. So, I did that and I let her (and my son) have a few less nutritious snacks during the week. At her next appointment she had gained 3 pounds and had rebounded (that was over a 3 month time period).
Couple of things to keep in mind: 1. Are you currently giving your child the same diet you follow? Meaning are you following a low-fat diet and he is having the same. Although a low fat diet is great for us, for a toddler, they need the fat for brain growth. So, with a good balance of veggies and fruit and protein - the more fat the better.
Second: one of the main reasons that my pediatrician decided her size was not a problem is that at the second appointment I took my son. My son, who is a healthy 4 year-old, only just graduated to a 2T in pants (believe it or not, but when he potty trained at 2, we had to go back to 12 month pants for waist size and he still can fit a long 18 month pant). He is small for his age as I and my husband were at that age. My daughter is the same. Heredity is going to play an important factor in your child's development. In fact, my Mom was told that my brother was going to be a midget - yet he is 5"10" now. If you come from a family of more petite people, who tend to grow later in their teens, then the fluctuation in weight or heighth is normal for your family.
Good luck and I suspect you will see a change in his weight at the next appointment, with no worries! BTW - I think your son weighs as much as my 2 and 1/2 year-old daughter!!!! :)