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.
She needs to realize some kids are just tall and skinny. My son is.
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 .
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