My DD is between the 0.4 and 2 percentiles, having been born there, gone up to the 25th, then down again, and we've had a LOT of hassle about her weight, which in my mind was totally pointless and verging on the irresponsible, as it has caused us huge amounts of stress and upset.
No child is supposed to exactly follow the charts. Nobody knows why, but breastfed babies have different growth rates from formula fed babies, as I am sure you know, and bf babies slow down after six months (our DD even earlier!). I wish I had been more questioning with the HV when the issue was first raised. I will never implicitly trust a HV again. We were referred to hospital, where we were told by the pediatrician to add butter, cheese, etc, which we were already doing, and also try an extra meal or snack (seven years of medical school for that?), but seeing as she's not a hugely hungry babe, that seemed to me like a useless bit of advice. She is perfectly healthy, just small.
In desperation, we just got a book, My Child Won't Eat, by Carlos Gonzalez. It's a La Leche League book, and TBH, at first I was quite resistant to reading it, as I find the LLL agenda a bit much for me, but there's a lot of common sense in it, and there's a lot in it about how our expectations, as parents, of what kids eat, is not realistic. As I understand it, they don't really need to eat that much once they get a bit older - he will probably be cutting down on what he eats, whether you like it or not. Basically, as long as you are OFFERING your son food, chances are he is growing properly, he's just growing the way he needs to be, and not the way the stupid charts think he should. I know it's disconcering that they go from eating all the time, to not being interested, but within reason, maybe we need to trust our kids know what they need.
Sorry if this is a bit of a rant, but if I can help someone else avoid the stress we have suffered, it would really cheer me up!