My son also got very bad colic & mild reflux when he was 1 month old. He also drink alot and was pretty big in size. And the pediatrician commented if he is doing well then leave it. Now my son who is coming to 2 yrs still fall in the category of the 95-97 percentile but he is doing real fine. His weight gain also slow down, maybe he is now active plus he is teething. So when they start to crawl and walk... all these weight issue will not be an issue. Ignore all your relatives comment.
If your hubby's aunty trying to introduce anymore things... Just ask her "Is it the best"... Tell her you only give your son the best? Then follow by asking her, proven by the pediatrician? Ask her as many question as possible.... until she piss off... this people tends to be pissed off when someone keeps questioning them, they will feel insulted.... she will then say "you dont't regret huh"...
You must let her know as a new mum you might be in-experience... but you got a mouth to ask your pediatrician... and let her know the pediatrician will know what is suitable for your dear son... she just got experience with her only daughter... and tell her even she reads alot she also not a qualified pediatrician, so her comments can only be reference but not a must to follow.
Tell your grandmum in law, even our fingers are not of the same length, does she think human will have the same body system... so dont't act smart. Her granddaughter might be 3 yrs apart from your dear son...but they came from different mother womb.