If the health professionals involved in OP's LOs care are not worried then why should we be? As OP has mentioned before her baby has been gaining average amounts of weight; he was just born smaller; the charts are drawn up with the assumption that babies born more petite will gain more than average in order to 'catch up' but this isn't always the case. Just as a baby born on the 98th centile isn't going to necessarily gain 1lb every week from then on because they were born big.
I have had four boys and have tens of nephews, nieces and small cousins in both mine and OHs family, one of my boys I switched to formula because he was reacting seriously to something in my milk and stopped gaining weight but while his allergies resolved on formula he didn't gain weight very well at all-he went through a chunky phase at the age of 2-3 but stopped gaining weight, lost some after an illness and is struggling to put on weight once again. My other son who had the same issue but who was deathly allergic to milk products and so had to be BF until 12 months, didn't gain much in his first year but after about 7 months of age started piling on the weight and has always been on the 50th to 75th centile in weight and around the 75th in height ever since. Also my eldest who gained very little weight in his early days; was combi fed at first and it did sweet FA to help him gain weight better; the time he started gaining 8oz a week was the short period of time where I don't remember him having any formula top ups.
It is not that people are defensive of breastmilk to the detriment of babies; but that formula top ups are not always a solution to growth issues; in fact they very rarely are. Babies failing to gain loads of weight can be due to other underlying issues as well but the assumption is always they are 'not getting enough'. By the time it has been found that it was something else causing the lack of weight gain BF has usually been completely discontinued-there are loads of ladies who have had this experience on this forum. I would also recommend OP to get a referral to a paediatrician if only for her own peace of mind xx