If Dr. Says he is okay then tell mom Dr. is happy with weight. If you don't/cant go to Dr. then ask your school nurse....better to be concerned mom and find out then risk CPS invilvement.
I would tell your mom too that with the childhood obesity epidemic, normal weight kids are starting to look too thin as mist kids today are too heavy
53lbs at 6 is perfect ,I know grandmas are a bit know it all (been there done that) but just speak to her politly and tell her that he is fine weight wise .
In Europe the obesity is just getting worse now so for us being 53lbs at 6 is perfect .
She needs to realize some kids are just tall and skinny. My son is.
My daughter is 5 yrs old nd is tall nd is around 50 lbs. Calmly tell your mom that he is perfectly healthy nd there's no reason to worry.