Please remember that the growth charts are based on babies fed primarily formula. If your son is taking breastmilk to his liking and eating solids you offer him until he is no longer interested, you're doing well. Don't stress about it.
My 20 month old daughter is hovering just below 20 lbs at 31 inches. That puts her in the 2nd percencile for weight and 10th for height. She's smart, alert, eats tons of fruits, veggies, dairy, grains, and meats along with nursing 8+ times a day. She is also go-go-going all day long: running, dancing, playing, climbing, chasing after the cats and her big sister. If your son is becoming more mobile, it would definitely explain a drop of his normal curve.
As parents, we are not trying to have heavy babies--but to nurture the ones we have to be what their genes intended. You're doing a wonderful job. Keep up the good work!