M.,
I think the "charts" are more trouble than they are worth, to be honest. Rather than getting worked up over his percentages, here are some questions to consider:
*Does he seem happy and satisfied, or is he fussy and cranky all the time?
(What is his activity level like? An undernourished child may be lethargic and tired.
*Has he recently started walking or gotten more active? This is the normal age for babies to start moving around more. As a result, they start burning their baby fat and putting on more muscles. You might see them get more lean, start looking more like a sturdy toddle than a chubby baby. (My first didn't gain any weight from 12-18 months, but she grew 5 inches. She dropped from the 95th to the 40th percentile for weight.)
*Has he learned a new skill recently? Often kids are so interested in practicing a new skill that they don't want to eat. It will pass.
*What body types do you and your husband have? Now is around the time your son's genetic blueprint will start kicking in. If you and/or your husband are petite, it is not reasonable to expect that your son will stay at the top of the charts.
Another consideration is that breastfed babies have a different way of growing that formula fed babies (who most growth charts are based on). Formula fed babies tend to gain weight at a slower, but steady rate. Breastfed babies typically gain more weight in the first six months of life, then slow or stop gaining in the next six to twelve months. This is normal!
A final thought - look at the chart mathematically. It represents 100% of the population (theoretically!). In that population, someone has to be the heaviest, and someone has to be the lightest. Where you fall on the chart is NOT an indicator of how healthy you are. Even experiencing a sudding jump or drop in percentage does not mean that something is wrong. It only means that something COULD be wrong.
My best advice is to keep nursing and make sure your son has plenty of highly nutritious snacks available to eat. Don't fill him up on junk food or lots of highly refined carbohydrates. Offer plenty of high-protein snacks to build muscle. (My girl liked cottage cheese, cheese sticks, and meat.) Beyond that, just keep an eye on the way he is acting. A happy, active, strong child is obviously not wasting away.
Best of luck,
S.