Is it ok if a 9 year old weighs 75 pounds?

This answer is only from my knowledge, i did not get a PHD in science so please don't rely 100% on my advice. My son who is 15 weighs 100 pounds. So im pretty sure that 75 pounds for a 9 year old is fine. i recommend to get the 9 year old's bones X-Rayed or get a blood test. im late 9 and im 60 pounds but im a male. im sure 75 pounds is fine most people i meet are about 10 to 9 pounds heavier than me.