When the growth plates fuse in his arms and legs, he will be done growing. Your doctor can give you a good estimate of how close he is to that. Be thankful that he is tall and healthy. 6'5" is better than having him too small. Look at the men in your family and your husbands and you will get some idea of how tall your son will be.
Thank you Titchou and Ibake&pray for your responses.
His father is 6'4 which is close to normal. But his brother is 6'10" which is just too tall and he would hate to get that tall. So I am trying to find out if we have any options at this point to stop this growth.